Update PR-blocking system

Our current shared PR-blocking system is a little clunky. We block all PRs that are not opened by an org member or someone on a whitelist. This is fine, but if contributor numbers grow, we'd need to add more org seats or maintain the whitelist by updating it in the workflow via extra PRs.

We could add a check if a contributor is a GitHub collaborator for the repo, like we check if they belong to the org. That way we can add collaborators without adding org seats and without clunky PRs to update the whitelist in the workflow.

This check can replace the `ALLOWED_USERS` parameter altogether, so we can delete all logic around that.