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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[dduportal]

👏