Upgrade to Java 21

[codeforkjeff]

I took a quick stab at this and here's what I ran into immediately:

• jacoco needs to be upgraded to at least 0.8.12
• ehcache 3.10.8 seems to have problems with 21: even after opening up module access, someone reported further errors: The JVM is preventing Ehcache from accessing the subgraph ehcache/sizeof#73

There's probably other issues as well.

[tfmorris]

My solution to the EHcache issue was to just switch to using entries as the caching unit (instead of MB). It seems like that along with some guidelines about how big the entries are could give devops sufficient control.