Setup GitHub Actions to automatically make PRs when master updates

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Currently, upstream/master is automatically pulled into master every day by this action, but I still need to manually rebase pogohwh if there are any changes.

Would be nice to use something like this instead: https://github.com/vsoch/pull-request-action

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But ofc, Actions can't trigger other Actions as changes made by the a token are ignored by other Actions of that same token.

189a275 was of no use. Might try making a custom token later, but then I might end up having to manually merge things anyway so...