[Feature Request] Do not close PRs that have auto-merge activated

[larsxschneider]

Description: If a PR is enabled for auto-merge, then stale bot should never close the PR. Stale bot could add a comment to the PR saying that the PR is stuck in the auto-merge process.

Justification: Auto-merge is a convenient feature to merge PRs without waiting for the completion of a long CI run. However, if a CI test fails unexpectedly (e.g. because of flakiness), then the PR would never turn green and consequently auto-merge would not perform the merge. After some time the stale action would close the PR.

However, I believe PRs that have auto-merge activated should never be closed automatically because there was already a serious intention to merge the PR.

/cc @leonor-loureiro @dkunkler