[githubgen] PR feedback follow-ups

[mowies]

This PR

• adds unit tests here and there to get coverage to a slightly better level
• refactors many parts of the code to make it better testable
• refactors parts of the code to be a bit less performant but much more readable (IMO of course)
• adds new flags to make the tool more suitable for general usage instead of just being focused on OTel Collector Contrib
  • caveat: the flags for now still have defaults that set everything for OTel Collector Contrib (this should probably change in another follow up to keep this PR from exploding)
• adds githubgen back to the released packages in versions.yaml
• addresses all feedback comments from feat: add githubgen tool #639

[codecov]

Codecov Report

Attention: Patch coverage is 54.90196% with 46 lines in your changes missing coverage. Please review.

Project coverage is 50.19%. Comparing base (20a1947) to head (ce7f859).

Files with missing lines	Patch %	Lines
githubgen/main.go	15.38%	22 Missing ⚠️
githubgen/codeowners.go	70.68%	16 Missing and 1 partial ⚠️
githubgen/issuetemplates.go	61.11%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #655      +/-   ##
==========================================
+ Coverage   48.86%   50.19%   +1.32%     
==========================================
  Files          57       57              
  Lines        3381     3405      +24     
==========================================
+ Hits         1652     1709      +57     
+ Misses       1568     1534      -34     
- Partials      161      162       +1     

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