Upgrade to java 21

[hydrozoa-yt]

Proposed changes

This suggestion makes the project run on java 21, instead of the current 11. I tried to make as few changes as possible.

Pull Request type

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• Unit tests pass locally, after applying my changes
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation
• Any dependent changes have been merged and published in downstream modules

Further comments

I noticed that we create a GsonBuilder 3 different places. We should probably reuse the one in GsonUtils, since it is now non-trivial to initialize.

[lare96]

Ideally would like to save any Java changes for pre-release, and would like to ensure we upgrade to a LTR of Java