Use CodeCov for coverage reporting.

[MikeMcQuaid]

An alternate solution to #7827. We used to use them and then bailed due to lack of Azure Pipelines support. Looks like they support both Azure and GitHub Actions now and their gem seems significantly better maintained than Coveralls at this point.

• 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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