Deprecate GitHub-API

I'd like to open up the discussion around deprecating this package. GitHub itself maintains a great JavaScript client library in [Octokit](https://github.com/octokit/rest.js). I think due to the popularity of our library, it almost hurts the community since we're splitting efforts across multiple projects.

I propose we come up with a rough timeline to deprecating this package and formally recommending people move to Octokit. We could still take security fixes or certain bug fixes for a period of time.

Start by reviewing the issue discussion and the linked Octokit project to understand the proposed migration. Done would require agreement on a deprecation timeline, the period for security or bug fixes, and how this package will formally direct users to Octokit.

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