github: automatically close PRs from the master branch.

[OhmSpectator]

This GitHub Actions workflow detects pull requests originating from the master branch and automatically closes them. It also leaves a comment explaining the reason and guiding contributors to create a feature branch instead.

Closes: #4637

[eriknordmark]

LGTM

[OhmSpectator]

We have tested it in @rene's repo, and it fails with the permission error:

{
  "message": "Resource not accessible by integration",
  "documentation_url": "https://docs.github.com/rest/pulls/pulls#update-a-pull-request",
  "status": "403"
}
{
  "message": "Resource not accessible by integration",
  "documentation_url": "https://docs.github.com/rest/issues/comments#create-an-issue-comment",
  "status": "403"
}

rene#41

[uncleDecart]

@rene needs to enable permisions for actions in his repository, should be somewhere in settings

edit: not sure what kind of power our GHAs have, they should be able to write to repo and close PRs..., but someone with admin access should check it

[OhmSpectator]

Here is how it works

rene#52