[FLINK-35011][tests] Change MockDeserializationSchema's visibility to public to prevent compilation failure in kafka connector

[Jiabao-Sun]

What is the purpose of the change

[FLINK-35011][tests] Change MockDeserializationSchema's visibility to public to prevent compilation failure in kafka connector

https://github.com/apache/flink-connector-kafka/actions/runs/8553981349/job/23438292087?pr=90#step:15:165

Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.0:testCompile (default-testCompile) on project flink-connector-kafka: Compilation failure
Error:  /home/runner/work/flink-connector-kafka/flink-connector-kafka/flink-connector-kafka/src/test/java/org/apache/flink/streaming/connectors/kafka/FlinkKafkaConsumerBaseTest.java:[60,39] org.apache.flink.streaming.util.MockDeserializationSchema is not public in org.apache.flink.streaming.util; cannot be accessed from outside package

Brief change log

This problem was introduced by #24432.
Rollback the MockDeserializationSchema's visibility to public to prevent compilation failure in kafka connector.

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change is already covered by existing tests.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not documented)

[flinkbot]

CI report:

• 14bc90d Azure: SUCCESS

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[Jiabao-Sun]

Hi @MartijnVisser.
Sorry for the issues introduced in #24432.
Please help review this PR when you have time.

[GOODBOY008]

LGTM