Ban JUnit 4 imports

[froque]

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.

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