feat: Refactor repositories download contents

[stevehipwell]

This PR refactors the behaviour of DownloadContents & DownloadContentsWithMeta with the former now being a direct passthrough to the latter as the only difference was the signature. The code has been refactored to use the API directly instead of via an unnecessary layer of indirection.

I've added an OpenAPI update to this PR as it proves that the updated code works against GitHub.

This change is required for #4151.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.77%. Comparing base (44908ea) to head (eaa3b4a).

Files with missing lines	Patch %	Lines
github/repos_contents.go	83.33%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4153      +/-   ##
==========================================
+ Coverage   93.76%   93.77%   +0.01%     
==========================================
  Files         211      211              
  Lines       19701    19677      -24     
==========================================
- Hits        18472    18452      -20     
+ Misses       1031     1029       -2     
+ Partials      198      196       -2     

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