Use JDK 21

[fractalwrench]

Goal

Uses JDK 21 to build the project on CI, bumping from 17. This gets the project closer to the latest LTS (AGP doesn't quite support it yet). I noticed there are a couple more warnings about targeting 1.8 bytecode when building the project, but functionally everything seems to work ok.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.97%. Comparing base (61d8c47) to head (57a616a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1317   +/-   ##
=======================================
  Coverage   63.97%   63.97%           
=======================================
  Files         154      154           
  Lines        3103     3103           
  Branches      304      304           
=======================================
  Hits         1985     1985           
  Misses       1038     1038           
  Partials       80       80           

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