chore: bump module to Go 1.22

[tri-adam]

Bump module Go version to 1.22, and drop testing against Go 1.21. This should (finally) allow updating the github.com/sigstore/sigstore dependency.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.42%. Comparing base (dae7b25) to head (9a07943).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
+ Coverage   62.80%   65.42%   +2.61%     
==========================================
  Files          37       37              
  Lines        3248     3248              
==========================================
+ Hits         2040     2125      +85     
+ Misses       1043      971      -72     
+ Partials      165      152      -13     

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