refactor!: Change GetRulesForBranch to ListRulesForBranch

[gmlewis]

BREAKING CHANGE: RepositoriesService.GetRulesForBranch is now RepositoriesService.ListRulesForBranch.

Relates to: #4117.

[codecov]

Codecov Report

❌ Patch coverage is 25.92593% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.45%. Comparing base (d9f9997) to head (0aa5a2f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
github/repos_rules.go	25.92%	39 Missing and 21 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4229      +/-   ##
==========================================
- Coverage   97.75%   97.45%   -0.31%     
==========================================
  Files         189      189              
  Lines       19035    19115      +80     
==========================================
+ Hits        18608    18628      +20     
- Misses        231      270      +39     
- Partials      196      217      +21     

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[gmlewis]

cc: @stevehipwell - @alexandear - @zyfy29 - @Not-Dhananjay-Mishra - @munlicode

[gmlewis]

Note that I'm not terribly concerned about the code coverage report in this PR since the JSON custom unmarshaler is tested separately.

[stevehipwell]

This looks good, just a minor question.

[stevehipwell]

I can see why this iterator isn't being generated, but do you need to add some configuration to block the generator or will it see that this already exists?

I'd also like a go refactoring it once it's merged as I think we could make the API a bit more specific and remove any.