Add official support for Java 21

[ecdye]

This also adds checks to make sure that Java 21 is installed before upgrading to openHAB 5.

Fixes #1937

[Larsen-Locke]

zigbee2mqtt test is disabled here. Couldn't we enable it back?

[ecdye]

zigbee2mqtt test is disabled here. Couldn't we enable it back?

Yep!, I just forgot to after we merged the fix. I'll do that now.

[ecdye]

@mstormi I can't quite fathom why, but OpenJDK 21 really doesn't like the GitHub Actions env. When I run it locally on Docker it works perfectly but not in Actions. I feel confident enough with all of this for it to be reviewed though.

I'm on the fence about defaulting to the Temurin over OpenJDK for 21 because of the issues I'm seeing here and because then it has to use unstable for the source. It also upgrades other packages when installing from unstable because they are dependencies which I don't really like (ie libc6). Temurin is also now the most widely supported JVM in general that Oracle doesn't build reference implementations anyways.

Let me know your thoughts.

(There ended up being a bunch of random fixes to automated testing that I found and added while debugging that stupid OpenJDK 21 issue that I never could figure out)