Auto-close dependabot PRs on envoyproxy repos other than envoy

### Description

Currently dependabot opens PRs on any mirror repos of envoyproxy/envoy.

This is annoying both for users forking the repo, and for mirror repos under the envoyproxy org

There is not an easy solution to this as i understand it, other than maintaining patch commits to remove the dependabot config etc

One possible solution wrt to mirror repos in the envoyproxy domain is that we add a gh action to auto-close these - this is what we currently do manually