chore: require go 1.21

[toddbaert]

Require go 1.21.

1.20 has long been EOL: https://endoflife.date/go

This is consistent with our guidelines on runtime support.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.89%. Comparing base (0b13f8a) to head (c5d839c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #294   +/-   ##
=======================================
  Coverage   81.89%   81.89%           
=======================================
  Files          10       10           
  Lines        1215     1215           
=======================================
  Hits          995      995           
  Misses        199      199           
  Partials       21       21           

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[trungdlp-wolffun]

@toddbaert

Did you forget to update README.md?

Also, Go 1.21 is no longer supported.