Separate auto-deploy cherry picking from preparation branch cherry-picking

Fully addresses the technical debt outlined in https://gitlab.com/gitlab-org/release-tools/-/issues/455, in order to more easily facilitate gitlab-com/gl-infra/delivery#943 (closed).

This merge requests builds on !1052 (merged) by customizing the two CherryPick::AutoDeployService and CherryPick::StableService classes, and further splits CommentNotifier in two so that we can remove hacks that were added at the time to support interoperability when working on an auto-deploy branch rather than a preparation MR.

Closes https://gitlab.com/gitlab-org/release-tools/-/issues/455