[JENKINS-71957] Fix support for Java 21

[alecharp]

Description

See JENKINS-71957.

When building the plugin with Java 17 and less, everything is fine but once building with Java 20+, there is a NPE occurring in the test.

This is because when testing with invalid setup, the exception is supposed to be printed out. However, the way the exceptions are printed changed. More specifically, the way a lock is acquired in order to print the exceptions. This should have no consequences on any code, as it is internal change of the JVM, but in the current plugin it creates a NPE are the object used to get the lock is a mock which is not properly initialized in the test class.

Testing

I ran mvn clean verify with Java 17 an no issue happened.
I ran mvn clean verify with both Java 20 and 21 and then both JiraCreateReleaseNotesTest.failBuildOnErrorEmptyProjectKey and JiraCreateReleaseNotesTest.failBuildOnErrorEmptyRelease failed with

NullPointer Cannot enter synchronized block because "lock" is null

With this changest, this is not occuring anymore.

To run this and validate the changes, the new Jenkinsfile needs to be used, so a maintainer or an admin must re-open the pull request.

🚨 Please review the guidelines for contributing to this repository.

• Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

[nit] The only real reason to make this a field in the first place was to use the @Mock annotation on it, so now that @Mock is being deleted this could be inlined and would be arguably more readable that way. But I do not feel strongly either way.

[alecharp]

Thanks @rantoniuk. Do you think you could release the plugin? Thanks