Ban JUnit 4 imports

[strangelookingnerd]

Ban JUnit 4 imports

To prevent regressions when adding new tests, https://github.com/jenkinsci/plugin-pom/pull/1178 introduced a new flag that enables a Maven Enforcer rule banning org.junit.* imports while allowing org.junit.jupiter.*.

With this change, the build will fail if any org.junit.* imports are introduced.

Testing done

None. Rely on ci.jenkins.io.

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[strangelookingnerd]

@jenkinsci/postbuildscript-plugin-developers Kindly requesting a review.