Ban JUnit 4 imports

[strangelookingnerd]

Ban JUnit 4 imports

To prevent regressions when adding new tests, jenkinsci/plugin-pom#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

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• 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]

[2025-08-15T12:42:38.331Z] Rule 0: org.apache.maven.plugins.enforcer.RestrictImports failed with message:
[2025-08-15T12:42:38.331Z] 
[2025-08-15T12:42:38.331Z] Banned imports detected in TEST code:
[2025-08-15T12:42:38.331Z] 
[2025-08-15T12:42:38.331Z] Reason: Use JUnit 5 (org.junit.jupiter.*)
[2025-08-15T12:42:38.331Z] 	in file:///home/jenkins/agent/workspace/Plugins_email-ext-plugin_PR-635/src/test/java/hudson/plugins/emailext/ExtendedEmailPublisherTest.java
[2025-08-15T12:42:38.332Z] 		org.junit.ClassRule 	(Line: 64, Matched by: org.junit.**)

Good catch, this Rule can be migrated to JUnit5. I'll post a PR shortly.

[strangelookingnerd]

@slide please reopen. PR got auto-closed due to my comment in #636

[slide]

Can you fix the conflicts?

[strangelookingnerd]

Can you fix the conflicts?

There are no conflicts

[slide]

It shows "This branch cannot be rebased due to conflicts"