Add go v1.21 to tests

[vorlif]

Summary

• Remove support for Golang v1.17 and v1.18
• Add Golang v1.21 to the tests
• Lint customizations

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (a1a735e) 87.70% compared to head (0a9579c) 87.76%.

Files	Patch %	Lines
catalog/poplural/ast/scanner.go	66.66%	1 Missing and 2 partials ⚠️
catalog/po/encoder.go	66.66%	1 Missing ⚠️
catalog/po/scanner.go	66.66%	1 Missing ⚠️
catalog/poplural/ast/parser.go	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   87.70%   87.76%   +0.05%     
==========================================
  Files          43       43              
  Lines        4058     4053       -5     
==========================================
- Hits         3559     3557       -2     
+ Misses        356      353       -3     
  Partials      143      143              

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