[FR] Auto-delete outdated PRs

Thanks for making CompatHelper :)

There's a minor issue I see coming up a lot for my workflow. After CompatHelper creates a PR, I'll independently make some change that makes the PR irrelevant. Maybe I make the same update it's requesting, or remove the dependency altogether. Closing a PR is not hard, but it takes enough clicks that I tend to put it off. The end result is a list of outdated PRs.

The obvious solution is to stop using CH (since I'm too-often ignoring it anyway). Honestly, to me I think the most valuable thing it adds is the automated alert that things are out of date. The (again, very minor) issue comes up for repos that are changing very quickly anyway. For those that are more in "maintenance mode", I think it's functionality is a great fit as is.

Anyway, it would be great if CH could occasionally check whether its current PRs are still relevant, and delete any that are outdated.