Move to Java 21

[nbaars]

Since this release is now out, we should update. This includes:

• pom.xml
• add enforcer rules
• update all Github actions
• update Dockerfile

[zubcevic]

Hi the Docker base image was already set to Java 21. I see you changed it to the temurin JRE version. But that will prevent solving the solution where your code gets compiled. See #1677 comments

[nbaars]

The release failed, due to missing ARM image not sure if that really was the issue though. Will fix it