HDDS-9506. Remove JUnit4 dependency

[adoroszlai]

What changes were proposed in this pull request?

Remove dependency on JUnit4.

Some modules still get JUnit4 as a transitive dependency (required by third-party libraries), so keep banning the import of JUnit4 classes. The ban now applies to all Ozone modules.

Remove JUnit4-specific items we no longer need:

• JUnit5's runner for JUnit4 tests (junit-vintage-engine)
• custom timeout rule (JUnit5AwareTimeout) added in HDDS-9361
• checkstyle rule that instructed developers to use JUnit5AwareTimeout
• allow.junit4 property, used to selectively disallow JUnit4 as we progressed with the migration

Tweaked banned imports:

• allow org.junit.platform, which is from JUnit5
• ban MiniDFSCluster (mini cluster from Hadoop); it was used accidentally by TestVolumeSet for some utility method (removed by using @TempDir)
• ban some test utils from Hadoop for which Ozone has alternative

There were two executions of maven-enforcer-plugin:

• banned-rocksdb-imports intended to restrict RocksDB classes, but got extended over time with other restrictions; this is overridden (relaxed) in rocksdb-checkpoint-differ
• ban-junit4-imports restricted JUnit4 classes
  This PR moves non-RocksDB restrictions to the second execution (to apply them in rocksdb-checkpoint-differ, too). The execution is renamed to ban-imports since it's now more generic.

Mention the source of original MetricsAsserts code in javadoc comment, as suggested by @kerneltime in #5987.

https://issues.apache.org/jira/browse/HDDS-9506

How was this patch tested?

CI:
https://github.com/adoroszlai/ozone/actions/runs/7563582590

[hemantk-12]

Thanks @adoroszlai for the patch.

LGTM.

Also thanks for driving the whole JUnit migration and finally seeing it to completion.

[kaijchen]

LGTM, thanks @adoroszlai.

[adoroszlai]

Thanks @hemantk-12, @kaijchen for the review.