Test with Java 21

[MarkEWaite]

Test with Java 21 on Linux

Prepare for future release of Java 21 by testing with a Java 21 early access build.

Testing done

Confirmed automated tests pass.

Submitter checklist

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