Test with Java 21

[MarkEWaite]

Test with Java 21

Java 21 was released Sep 19, 2023. We want to announce full support for Java 21 in early October 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 we believe that 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.

Also updates parent pom to most current release and removes unneeded hamcrest dependency declaration.

Supersedes pull request:

• 325

Testing done

Confirmed that 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

[MarkEWaite]

@gmcdonald or @olamy would you be willing to merge this pull request so that Java 21 is regularly tested with the plugin. No need for a release, just merging it to the main branch is enough.

[gmcdonald]

sure thing!