ci: use Go 1.21

[tdakkota]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.78%. Comparing base (6383038) to head (8bf8791).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1189   +/-   ##
=======================================
  Coverage   73.78%   73.78%           
=======================================
  Files         188      188           
  Lines       12661    12661           
=======================================
  Hits         9342     9342           
  Misses       2785     2785           
  Partials      534      534           

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