Run tests on JDK 21

[msridhar]

It looks like most of our tests pass on JDK 21. Exceptions are the JSpecify generics tests, which rely on an API that changed in JDK 21 (see #827), and the JarInfer tests, as it looks like WALA does not yet support running on JDK 21 (see #829). Core NullAway support (ignoring the experimental/WIP JSpecify mode) should be working. This PR adds test configs so that we run tests on JDK 21 wherever possible. We update our Jacoco and Mockito versions to make this work.

We also take the opportunity to move some common test configuration code to the nullaway.java-test-conventions.gradle file, which previously just held configuration for Jacoco.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01% 🎉

Comparison is base (fcaf39e) 86.75% compared to head (bcabfb9) 86.77%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #826      +/-   ##
============================================
+ Coverage     86.75%   86.77%   +0.01%     
  Complexity     1870     1870              
============================================
  Files            74       74              
  Lines          6178     6178              
  Branches       1202     1202              
============================================
+ Hits           5360     5361       +1     
  Misses          406      406              
+ Partials        412      411       -1     

see 1 file with indirect coverage changes

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[msridhar]

Converted this one to a draft again as I'd like to get rid of at least some of the copy-pasting in the Gradle build files

[msridhar]

This is now ready for review again

[lazaroclapp]

Minor nit, but LGTM otherwise!