NEWS: ZGC of Java 21 now used by default

[Makarius]

*** System ***

* The Z Garbage Collector (ZGC) of Java 21 is now used by default (see
also https://wiki.openjdk.org/display/zgc). This should work uniformly
on all platforms, but requires a reasonably new version of Windows
(Windows 10 or Windows Server 2019).

This refers to Isabelle/1a4be2516f50.


I've been testing this locally for quite some time, now it is the default. 
Technically, it has become possible, because our old Windows Server 2012 
(vmnipkow9) has been discontinued.


	Makarius