Add Java 21 for linux

[timja]

(not touched Windows in this PR as it's not there for 17 yet, Windows in general needs some work and can be a follow up)

I've added the tag preview to the end as the Java version isn't GA'ed yet. This can be released once we ship an official Java version.

Testing done

Built all new images locally and ran them.
Tests pass on CI

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Beta Give feedback

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

[dduportal]

👏