Add Java 21 to GitHub testing matrix

[JonathanLennox]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.93%. Comparing base (7431511) to head (c941642).
Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1192      +/-   ##
============================================
- Coverage     27.27%   26.93%   -0.35%     
+ Complexity      489      475      -14     
============================================
  Files           125      125              
  Lines          8084     8086       +2     
  Branches       1121     1122       +1     
============================================
- Hits           2205     2178      -27     
- Misses         5597     5625      +28     
- Partials        282      283       +1     

see 2 files with indirect coverage changes

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