OCPBUGS-25989: go.mod: bump to golang 1.21

[stevekuznetsov]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9ffba44) 42.64% compared to head (9a02b66) 42.64%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #311   +/-   ##
=======================================
  Coverage   42.64%   42.64%           
=======================================
  Files          42       42           
  Lines        3583     3583           
=======================================
  Hits         1528     1528           
  Misses       1903     1903           
  Partials      152      152           

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