Java 21 for next major release?

Given Java 25 is LTS, we could move from Java 17 to 21 in Spoon.

Benefits:

— new language features, pattern matching, getFirst...
— remove deprecated APIs like the old unused Javadoc API?
— Cleanup CI to not use 17 anymore.

There is an argument to wait for Maven 4, but I have seen no release timeline. 

@SirYwell @I-Al-Istannen maybe you are also aware of some APIs we want to drop finally