[isabelle-dev] NEWS: Update to OpenJDK 21

[Makarius]

*** System ***

* Update to OpenJDK 21: the current long-term support version of Java.


This refers to Isabelle/4fb5e6499da9.

Official release-notes: 
https://docs.azul.com/core/zulu-openjdk/release-notes/21-ga


A quick test looks good so far.

Moreover, it might be time to change a few fundamental Java runtime defaults, 
e.g. ZGC https://openjdk.org/jeps/439


	Makarius