OpenJDK 21

Hi,

I'd like to update java-common to OpenJDK 21, basically uploading the 
package from experimental to unstable.  In the past, Emmanuel was 
leading these updates, but he told me privately that he doesn't have 
much time in the near future doing that.  The transition was finished 
for Ubuntu earlier this year (thanks Vladimir), and most, if not all 
patches should be in the Debian bug tracker.

We will have to keep m68k as pointing to 17 for now.

I don't want to drive this transition on my own (besides the java-common 
upload), I still have to do some other transitions.

Matthias

PS: I'll be at DebConf, but I doubt we'll have much attendance from the 
debian-java team there.