Upgrade to Java 21

[bpkroth]

Separates a conversion to Java 21 from #398 and also handles errors encountered in #369.

[bpkroth]

@apavlo staging this as a smaller change, but there's a few other related ones.
Main concern here is that it is potentially a breaking change for folks using just the jar release, so we should probably also tag a new release commit once this is merged.
This change also incorporates a new version tag in the relevant files to match that.
Let me know if you have any other thoughts.

[bpkroth]

@apavlo staging this as a smaller change, but there's a few other related ones. Main concern here is that it is potentially a breaking change for folks using just the jar release, so we should probably also tag a new release commit once this is merged. This change also incorporates a new version tag in the relevant files to match that. Let me know if you have any other thoughts.

Discussed offline. Gonna merge this now and tag a new release. Thanks!