[Request] Upgrade to Quarkus 3.8 LTS and JDK 21

[andrewazores]

Describe the feature

Since https://github.com/cryostatio/cryostat3/pull/352 CI only runs for JDK 17, since there are compatibility issues with JDK 21 arising in these newer releases. Our actual distributed builds are only using JDK 17 and the checks for 21 were to keep on top of the eventual upgrade.

Once Quarkus 3.8 builds are available downstream we should upgrade our upstream dependency to 3.8.x, and this should also support JDK 21 well so it would be a good time to make that upgrade too (and drop JDK 17).

See also #363 #179

Anything other information?

No response