chore: bump module Go version to 1.21

[tri-adam]

Bump module Go version to v1.21. Use slices package to simplify implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 65.26%. Comparing base (dd1e34c) to head (df206d4).

Files	Patch %	Lines
pkg/integrity/select.go	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #351      +/-   ##
==========================================
- Coverage   65.27%   65.26%   -0.01%     
==========================================
  Files          34       34              
  Lines        3266     3245      -21     
==========================================
- Hits         2132     2118      -14     
+ Misses        978      974       -4     
+ Partials      156      153       -3     

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[dtrudg]

Hold an a sec please - need to verify something wrt build that don't have access to Go 1.21 yet.

Go ahead... we are month after 1.20 sunset, and I don't see movement on EPEL / EL. We shouldn't be beholden to them for extended periods of time.