Upgrade to JUnit Jupiter 5.5.0-RC2

[snicoll]

This also gives us an opportunity to simplify the build setup required to exclude the vintage engine now that junit-team/junit5#1872 has been fixed.

[snicoll]

The upgrade has broken the build:

org.junit.platform.commons.PreconditionViolationException: Failed to register extension via @RegisterExtension field [private org.springframework.boot.cli.CliTester org.springframework.boot.cli.RunCommandIntegrationTests.cli]: field must not be private.
	at org.junit.platform.commons.util.Preconditions.condition(Preconditions.java:296)
	at org.junit.jupiter.engine.descriptor.ExtensionUtils.lambda$registerExtensionsFromFields$3(ExtensionUtils.java:103)
	at java.util.ArrayList.forEach(ArrayList.java:1257)
	at org.junit.jupiter.engine.descriptor.ExtensionUtils.registerExtensionsFromFields(ExtensionUtils.java:102)
	at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.instantiateAndPostProcessTestInstance(ClassBasedTestDescriptor.java:262)
	at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$testInstancesProvider$2(ClassBasedTestDescriptor.java:247)
	at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$testInstancesProvider$3(ClassBasedTestDescriptor.java:251)
	at java.util.Optional.orElseGet(Optional.java:267)
	at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$testInstancesProvider$4(ClassBasedTestDescriptor.java:250)

[wilkinsona]

CliTester needs to be registered as an extension as it's a before each and after each callback.

[snicoll]

RC2 is out in the meantime so let's reuse this issue to update to it