refactor: use simple-github for detecting branches

[ahal]

This allows us to use app auth in addition to token auth.

[ahal]

I realize that this isn't really simplifying much. I'm mostly doing this because I want to stand up some automation that will require auth here, and I want to stop relying on the moz-automation Github accounts and start using app auth for everything instead.

[ahal]

Also I filed mozilla-releng/simple-github#49 to add implicit support for pagination in simple-github. It'll get there eventually :p

[bhearsum]

Thanks for this enhancement/follow-up. Consider updating the GITHUB_TOKEN warning to also mention app configuration. (I could go either way on this...it's not something someone is likely to use locally, but I suppose it could conceivably come up in some future CI configuration.)

[ahal]

Thanks, I think app auth is complicated enough that it'll probably just add confusion. I think we should just omit it from the warning.