utils/github: fix too_many_open_prs?

[carlocab]

• 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 typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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This doesn't count PRs from forks, so it only works correctly for
maintainers (and only if they use non-fork branches).

Let's fix that, and:

• simplify the logic by using paginate_graphql
• return early if it's impossible for them to have too many open PRs

[MikeMcQuaid]

Makes sense, thanks @carlocab!