[isabelle-dev] Update to OpenJDK 21.0.2

[Makarius]

With Isabelle/667cb8b79909 we are on OpenJDK 21.0.2, see also the "January 
2024 Quarterly Update Release Notes" from Azul: 
https://docs.azul.com/core/release-notes/release-notes

There are many small changes: relevant for us is anything tagged "21" and "PSU".


I am very glad that Isabelle has only one linear timeline of official 
releases, and no "patches" for older releases (and no "branches" whatsoever). 
Thus we can spare several orders of magnitude of resources for maintenance and 
development.


	Makarius