Remove circe-generic-extras

[fthomas]

This concludes the work started in #3514 and #3515 to remove the dependency on circe-generic-extras.

Here are some notes about the changes:

• Resolver cases had non-optional fields with defaults that needed to be made optional so that the derived decoder does not fail if those are not provided
• ArtifactChange and ArtifactChanges could have always used deriveDecoder since the case classes have no defaults
• ScalafixMigrations had a default value but a scalafix-migrations.conf without a migrations field makes no sense to me and it should be the same as ArtifactChanges anyway

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.73%. Comparing base (a31586b) to head (0f3ab16).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3517      +/-   ##
==========================================
+ Coverage   89.63%   89.73%   +0.10%     
==========================================
  Files         171      171              
  Lines        3540     3536       -4     
  Branches      331      321      -10     
==========================================
  Hits         3173     3173              
+ Misses        367      363       -4     

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