chore: fix tests for updated Schema Registry client internals

[vcrfxia]

Description

Some unit tests are failing in the build after confluentinc/schema-registry#2674 was merged, due to improper usage of mock SR clients on the ksql side. We should be using the provided MockSchemaRegistryClient implementation rather than Mockito.mock(SchemaRegistryClient.class) since the methods which need to be mocked when using the latter are implementation details of SR which are subject to change.

The specific failure in this case is that a new internal method added to the SchemaRegistryClient usage is resulting in an NPE in our unit tests because the unit tests do not provide mocked implementations for this new method. Here's an example failure from KsqlJsonSchemaDeserializerTest.shouldDeserializeJsonObjectCorrectly:

Error serializing message to topic: bob. Converting Kafka Connect data to byte[] failed due to serialization error of topic bob: 
Hint: You probably forgot to add VALUE_SCHEMA_ID when creating the source.
	at io.confluent.ksql.serde.connect.KsqlConnectSerializer.serialize(KsqlConnectSerializer.java:56)
	at io.confluent.ksql.serde.tls.ThreadLocalSerializer.serialize(ThreadLocalSerializer.java:37)
	at io.confluent.ksql.serde.json.KsqlJsonSchemaDeserializerTest.shouldDeserializeJsonObjectCorrectly(KsqlJsonSchemaDeserializerTest.java:109)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.mockito.internal.runners.DefaultInternalRunner$1$1.evaluate(DefaultInternalRunner.java:55)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:100)
	at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:107)
	at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:41)
	at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:163)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
	at org.junit.vintage.engine.execution.RunnerExecutor.execute(RunnerExecutor.java:42)
	at org.junit.vintage.engine.VintageTestEngine.executeAllChildren(VintageTestEngine.java:80)
	at org.junit.vintage.engine.VintageTestEngine.execute(VintageTestEngine.java:72)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:107)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:114)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:86)
	at org.junit.platform.launcher.core.DefaultLauncherSession$DelegatingLauncher.execute(DefaultLauncherSession.java:86)
	at org.apache.maven.surefire.junitplatform.LazyLauncher.execute(LazyLauncher.java:55)
	at org.apache.maven.surefire.junitplatform.JUnitPlatformProvider.execute(JUnitPlatformProvider.java:223)
	at org.apache.maven.surefire.junitplatform.JUnitPlatformProvider.invokeAllTests(JUnitPlatformProvider.java:175)
	at org.apache.maven.surefire.junitplatform.JUnitPlatformProvider.invoke(JUnitPlatformProvider.java:139)
	at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:456)
	at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:169)
	at org.apache.maven.surefire.booter.ForkedBooter.run(ForkedBooter.java:595)
	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:581)
Caused by: org.apache.kafka.connect.errors.DataException: Converting Kafka Connect data to byte[] failed due to serialization error of topic bob: 
	at io.confluent.connect.json.JsonSchemaConverter.fromConnectData(JsonSchemaConverter.java:106)
	at io.confluent.connect.json.JsonSchemaConverter.fromConnectData(JsonSchemaConverter.java:87)
	at io.confluent.ksql.serde.connect.KsqlConnectSerializer.serialize(KsqlConnectSerializer.java:53)
	... 49 more
Caused by: org.apache.kafka.common.errors.SerializationException: Error serializing JSON message
	at io.confluent.kafka.serializers.json.AbstractKafkaJsonSchemaSerializer.serializeImpl(AbstractKafkaJsonSchemaSerializer.java:166)
	at io.confluent.connect.json.JsonSchemaConverter$Serializer.serialize(JsonSchemaConverter.java:175)
	at io.confluent.connect.json.JsonSchemaConverter.fromConnectData(JsonSchemaConverter.java:98)
	... 51 more
Caused by: java.lang.NullPointerException
	at io.confluent.kafka.schemaregistry.client.rest.entities.Schema.<init>(Schema.java:169)
	at io.confluent.kafka.serializers.AbstractKafkaSchemaSerDe.registerWithResponse(AbstractKafkaSchemaSerDe.java:507)
	at io.confluent.kafka.serializers.json.AbstractKafkaJsonSchemaSerializer.serializeImpl(AbstractKafkaJsonSchemaSerializer.java:126)
	... 53 more

This PR fixes the failing tests by updating our mock SR client usage to MockSchemaRegistryClient.

This PR also contains an additional fix for the SR dependency version: it was set to 7.6.0-0 in this PR which is a mistake -- setting the version to a -0 nanoversion means that Jenkins no longer automatically bumps the version as new nanoversions are released (here's an example of where this previously happened), which means ksql development locally will not match what's seen in Jenkins, since Jenkins continually recompiles the latest schema-registry whereas ksql developers typically do not.

Testing done

Describe the testing strategy. Unit and integration tests are expected for any behavior changes.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")
• Do these changes have compatibility implications for rollback? If so, ensure that the ksql command version is bumped.

[vcrfxia]

PR build made it past the test failures that this PR intends to fix. There are other unrelated failures in the ksql master build right now which failed the build. Going to merge to improve the situation even though it's still not green yet.