chore: migrate to java 21

[vil02]

This PR updates the used version of Java to 21. Similar to #2819.

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 38.59%. Comparing base (d77d901) to head (c93e720).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5163      +/-   ##
============================================
+ Coverage     38.58%   38.59%   +0.01%     
  Complexity     2379     2379              
============================================
  Files           516      516              
  Lines         15315    15315              
  Branches       2957     2957              
============================================
+ Hits           5909     5911       +2     
+ Misses         9121     9120       -1     
+ Partials        285      284       -1     

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