Enable branch protections and auto-merging for the repo

[msridhar]

Branch protections let you enforce rules on PRs before merging, like CI jobs must pass or there must be one approving review (the rules can be configured such that administrators can override them):

https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/defining-the-mergeability-of-pull-requests/about-protected-branches

Once branch protections are in place, an auto-merge functionality becomes available, where GitHub can automatically merge a PR once all rules have been satisfied:

https://docs.github.com/en/github/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/automatically-merging-a-pull-request

We recently enabled this on the WALA repository, and I've found the auto-merge functionality to be a nice productivity boost. When appropriate you can enable it on a PR, and then there is no need to come back to GitHub to click merge after the CI jobs have passed and the reviews have gone through. This might be nice to have for the Checker Framework repo as well.

[msridhar]

Looks like branch protections have been enabled