Use Java 21 instead in samples and templates

[octonato]

Investigate if there is anything blocking us to use java 21 in the samples and templates.

I did a first quick try with the code-first SDK and it just worked.

[octonato]

It seems that Protobuf samples are still using Java 11. Not sure if oversight or we have a reason for it.

[johanandren]

Akka 2.6 is not tested with 21 (no released Akka version is yet, but for main we have nightlies on 21 across Akka modules) so there may be a problems there we don't know of.

[octonato]

Ok, I open a PR to see what CI says, but we can wait to merge it if we feel it's too early for the move.

[aludwiko]

Looking forward to the merge :)

[octonato]

Done