Add .github to export-ignore in .gitattributes

[JurianArie]

Hi! I noticed that the workflow files are in my vendor folder. This will remove the files from distributions.

As a side note, I saw that Symfony does this with /.git* export-ignore, but I did not want to make changes to the existing entries like .gitignore.

Link to Symfony: https://github.com/symfony/http-foundation/blob/0edbbac68bfa4963b819363ad63a22078d1cd297/.gitattributes#L3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e6bc62d) to head (8334eb1).
⚠️ Report is 2 commits behind head on 9.x.

Additional details and impacted files

@@             Coverage Diff             @@
##                 9.x      #235   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        52        52           
===========================================
  Files              3         3           
  Lines            167       159    -8     
===========================================
- Hits             167       159    -8     

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