Install github app to automatically close PRs on github mirror

Hi,

I think the postgres github mirror should install an application that
automatically closes PRs. As there's no better way :(

I found https://github.com/dessant/repo-lockdown/ , which seems
suitable. Only requires read/write permissions for issues/PRs, and
permission to read .github/lockdown.yml. It can be configured in a
separate .github repository.

As an example, here's the config:
https://github.com/anarazel/.github/blob/master/.github/lockdown.yml
and an example PR
https://github.com/anarazel/postgres/pull/1

Greetings,

Andres Freund