JDK 21 (LTS) Upgrade

TODO:

- [x] Maven build deps updates.
- [x] Gradle build updates.
- [x] Fix Javadoc warnings.
- [x] Fix Gradle checkstyle
- [x] IntelliJ warnings / suggestions for JDK 21 (green ticks).
- [x] Sonar fixes.
- [x] Docker build.
- [x] Release!