Test with Java 21, require Jenkins 2.387.3 or newer

[MarkEWaite]

Test with Java 21, review Jenkins 2.387.3 or newer

Java 21 was released Sep 19, 2023. We want to announce full support for Java 21 in November and would like the most used plugins to be compiled and tested with Java 21.

The acceptance test harness and plugin bill of materials tests are already passing with Java 21. This is a further step to improve plugin readiness for use with Java 21 and for development with Java 21.

The change intentionally tests only two Java configurations, Java 17 and Java 21 because the risk of a regression that only affects Java 11 is low. We generate Java 11 byte code with the Java 17 and the Java 21 builds, so we're already testing Java 11 byte code.

This pull request also updates the minimum Jenkins version from 2.361.4 to 2.387.3. I'm willing to revert that change if it is a blocker that prevents this from being merged.

Also includes or supersedes pull requests:

• 269

• 268

• 262

• 261

• 256

Testing done

Confirmed tests pass with Java 21 on Linux.

Submitter checklist

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

[MarkEWaite]

@jglick or @vlatombe could you merge this so that we're testing with Java 21? No need for a new release, just merge