[Linux] docs: Update sponsors to say GitHub Actions not CircleCI

[issyl0]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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• We switched from Circle CI to Azure Pipelines, then to GitHub Actions.
• I can't find any directly linkable official GitHub logo assets on
  https://github.com/logos and I don't want to upload an image directly
  to this repo. So, remove the image. If that's acceptable.

[MikeMcQuaid]

Thanks again @issyl0!