Skip repos if their last commit is older than n

[fthomas]

Scala Steward is currently creating a lot of wasted PRs in abandoned projects. See for example https://github.com/circe/circe-argus which had the last commit in August 2023. Since then Scala Steward created 43 PRs of which 18 are open and 25 are closed (because they were superseded by newer updates). I would say that every new PR that Scala Steward creates in this repo is wasted as long as no human touches it again. To prevent such waste, I propose to to skip any repo whose last commit is older than some threshold. The threshold could be set in the repo config so that we can set a sensible default which could be overridden on a per-instance and per-repo basis.