Upgrade JDK?

[lintool]

We're still on JDK11.

Should we jump all the way to JDK 17 or JDK 21? These are the LTS choices:
https://www.oracle.com/ca-en/java/technologies/java-se-support-roadmap.html

[lintool]

My plan is to bump up to JDK 21. This will be my next big project.

[lintool]

See #2410

[tteofili]

+1, we would also get some benefits on the Lucene side (e.g., vector comparison speedups via Panama Vector API).
perhaps this might require some version bump?

[lintool]

Done!