Update to Java 21

[ilianoKokoro]

Consider updating to Java 21. I didn't get any issue when updating to this version.

[maxrave-dev]

Why we should update to new Jdk

[ilianoKokoro]

@maxrave-dev It is really not necessary, but it's just generally a good thing to update to more recent versions as they often bring build performance improvements, and it does not seem to require any change to the codebase. It's an easy change.