Remove Maestro's auto-merging capabilities & allow setting auto-merge on PRs

## Context & motivation

Maestro has the capability to auto-merge PRs for a very long time since before this feature was available in both GitHub and Azure DevOps.
Since this feature is now available, devs can configure auto-merge and the service does not need to do this. This means that the service technically won't need write access to all repositories. This would make the service pretty harmless and secure as stealing Maestro's secrets won't get you very far. This means the service won't possibly have to onboard to Service Layer Auditing to log its own effects.
Additionally, the service could be able to toggle the auto-merge on for PRs so that users won't have to do it.

## Goals

- [ ] Remove the auto-merge capability from Maestro
- [ ] Remove the `All checks successful` check as it will have no function anymore
- [ ] Remove the `No requested changes` check
- [ ] Make all remaining checks run always
- [ ] Remove the merge policies settings completely
- [ ] Add a new flag on subscriptions that will make Maestro set the auto-merge on the PR after creation
- [ ] Communicate the change out