close pull requests from forks' stable and staging (#42456)

close pull requests from forks' stable and staging (#42456)

* close on master workflow: add stable and staging to blacklist, tweak wording, update commented out text

* keep main and develop in blacklist

* critical grammar and formatting

* argh how even (bring back old version of commented-out section)

* thats not the right word

* only close when head repo is a fork

from github docs: https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=opened#pull_request