Automatic toggling of merge train based on mirror status

In #960 (closed), we allowed for a branch divergence of master between Canonical and Security by utilizing the merge-train project to merge the Canonical branch into its Security counterpart.

The divergence between mirrors still exists -- Canonical is unable to push master -- because the Security mirror still contains commits that Canonical doesn't have (the merge-train commit, and any potential security fixes).

When a security release is completed and we sync Security back to Canonical, the divergence should no longer exist and the built-in mirroring should be sufficient again. At this point the merge-train is unnecessary.

We should investigate if we can toggle the scheduled merge-train task depending on the status of the mirror.

Implementation

Have a scheduled release-tools task that runs regularly and:

• Checks the status of projects/gitlab-org%2Fgitlab/remote_mirrors API
  • If the last_error message contains refs/heads/master, we need the merge-train -- toggle the https://ops.gitlab.net/gitlab-org/merge-train/-/pipeline_schedules/110/edit task on via API
  • Otherwise, toggle the task off