CI: Migrate from Azure DevOps to Github Actions

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GitHub Actions run faster and more reliably. Their Web UI is better integrated in pull requests, particularly for retrying jobs.

Continuation of #1128.

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The "check" job, using re-actors/alls-green is now a required job for the master and stable branches

[moreati]

The Azure Pipelines ("mitogen-hq.mitogen") is no longer a required check.