Fix go directive version in go.mod

[mauri870]

According to https://go.dev/issues/62278 a concrete go version has to include the patch number or an additional toolchain directive. The project declares the version as 1.21 which is invalid and will result in an error:

GOTOOLCHAIN="go1.20+auto" go1.21.5 mod tidy
go: downloading go1.21 (linux/amd64)
go: download go1.21 for linux/amd64: toolchain not available

Changed to 1.21.0 as this is the first actual release of go 1.21.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.79%. Comparing base (1e295e0) to head (dc50d10). Report is 11 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   70.89%   72.79%   +1.89%     
==========================================
  Files          55       54       -1     
  Lines        8809     9655     +846     
==========================================
+ Hits         6245     7028     +783     
- Misses       1675     1732      +57     
- Partials      889      895       +6     

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