Update to jdk-21.0.6

[Makarius]

With Isabelle/82cf33956a17 we are on current jdk-21.0.6, which has been 
released this week on time for our own release. Any remaining support for old 
jdk-17 has been discontinued.

Details of the current PSU (Patch Set Update) are here: 
https://docs.azul.com/core/pdfs/january-2025/azul-zulu-ca-release-notes-january-2025-rev1.1.pdf 
--- I've checked all occurrences of "21", and it looks pretty stable and 
well-maintained.

An interesting side-remark in the changelog is about CRaC: 
https://wiki.openjdk.org/display/crac --- this will eventually provide 
something like a "dumped heap image" as we are accustomed to in Poly/ML. 
Welcome back to the old LISP Club, OpenJDK!


	Makarius