Add Java 21 to CI build matrix

[valfirst]

Add Java 21 (LTS) to CI build matrix

Checklist

• Read the contributing guide
• PR should be motivated, i.e. what does it fix, why, and if relevant how
• If possible / relevant include an example in the description, that could help all readers
  including project members to get a better picture of the change
• Avoid other runtime dependencies
• Meaningful commit history ; intention is important please rebase your commit history so that each
  commit is meaningful and help the people that will explore a change in 2 years
• The pull request follows coding style
• Mention Fixes #<issue number> in the description if relevant
• At least one commit should mention Fixes #<issue number> if relevant

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (9f06733) 85.51% compared to head (88e7b68) 85.53%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3145      +/-   ##
============================================
+ Coverage     85.51%   85.53%   +0.01%     
- Complexity     2908     2912       +4     
============================================
  Files           333      333              
  Lines          8852     8856       +4     
  Branches       1095     1095              
============================================
+ Hits           7570     7575       +5     
+ Misses          994      993       -1     
  Partials        288      288              

Files	Coverage Δ
src/main/java/org/mockito/ArgumentCaptor.java	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[valfirst]

Blocked by #3147

[TimvdLippe]

Seems like CodeCov is having some issues 😢

[TimvdLippe]

Seems like the upload token doesn't work on PRs: codecov/feedback#126

[valfirst]

@TimvdLippe it seems Android 33 job is flaky

[TimvdLippe]

Yeah unfortunately it is 😢 But the rest works, so let's merge this. Thanks for the work!