Update CI to use JDK 21

[anatoliykmetyuk]

No description provided.

[bishabosha]

I saw a lot of the same errors from the CI logs when I tried to test with JDK 21 locally, I guess some places are testing with older than 3.3.1 scala on the classpath - telltale sign:

class file /modules/java.base/java/lang/reflect/AccessFlag.class is broken, reading aborted with class java.lang.RuntimeException
bad constant pool index: 0 at pos: 5189

[bishabosha]

Anyone have ideas why community_build_[a/b] fail with this, but community_build_c works?

fatal: detected dubious ownership in repository at '/__w/dotty/dotty'
To add an exception for this directory, call:

	git config --global --add safe.directory /__w/dotty/dotty

[anatoliykmetyuk]

Most likely I won't have the time to get back to it soon, as I'm loaded these days with other tasks. So I'll close it FTTB, if anyone wants to overtake it, feel free to reopen it.