NUTCH-3145 Upgrade to JUnit 6

[lewismc]

This is a PR for NUTCH-3145. It is quite an interesting issue as JUnit 6 requires Java 17. I am not convinced/sure that we want to orchestrate an upgrade from Java 11 --> 17 through a JUnit upgrade. I opened this ticket as a bit of an experiment. It also encouraged me to check in on NUTCH-2987 and perhaps more importantly the behemoth that is HADOOP-17177.
Any feedback or thoughts welcome. Thank you

[sebastian-nagel]

I'd be conservative and would not yet force users to require Java 17 as runtime, esp. given that is not yet 100% supported by Hadoop. A backward-compatible approach, similar to that described in HADOOP-18887 might be better.

What about?

• Require Java 17 to build Nutch, resp. run the unit tests.
• But still allow a Java 11 runtime by setting javac.version=11. The binary package would then still run on Java 11.

This option seems to work (I tried it, but without exhaustive testing).

[lewismc]

Yes I agree with you @sebastian-nagel I'm not keen to force a JDK upgrade to 17 via this PR either. I'll augment the PR accordingly. Thanks for the review.

[lewismc]

I augmented the CI to accommodate the backward-compatible Java version strategy for Apache Nutch, similar to the approach described in HADOOP-18887.
Key principle:

• Build requirement: Java 17+ (required by JUnit 6)
• Runtime compatibility: Java 11+ (bytecode target)