jdk #21 support

[vladak]

Java SE 21 is a LTS release, we should consider switching to that. Might still want to keep the Java 11 build target to support existing installations. The transition will likely fix the build warnings in #4312.

[tarzanek]

I partially did the migration in #4570
(which compiles and runs tests with jdk 21 ... just tests fail, since I need to fix URL to URI compat and tests ... and review the ignoring of some of the warnings jdk21 fires)