Test with Java 21

[gounthar]

Java 21 was released Sep 19, 2023. We want to announce full support for Java 21 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 shallow. We generate Java 11 byte code with the Java 17 and the Java 21 builds, so we're already testing Java 11 byte code.

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
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• [X] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov[bot]]

Codecov Report

Merging #802 (70a1346) into master (2671fa5) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #802   +/-   ##
=========================================
  Coverage     72.86%   72.86%           
  Complexity     1032     1032           
=========================================
  Files            88       88           
  Lines          3825     3825           
  Branches        440      440           
=========================================
  Hits           2787     2787           
  Misses          892      892           
  Partials        146      146           

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