Handle branch divergence in prefixed security mirrors

As part of gitlab-org/release&14 (closed), we've got forks in the gitlab-org/security group using pull mirroring with a security/ branch prefix to stay in sync.

However, security fixes will be merged into security/master, causing a divergence. We want to avoid this using merge-train to bring the master branch into security/branch, either alerting loudly on conflicts, or favoring the version in security/master.

This issue continues #472 (closed)

changed milestone to %12.5

added priority1 ~9224530 + 1 deleted label

mentioned in issue #472 (closed)

mentioned in epic gitlab-org/release&14 (closed)

added teamDelivery label

mentioned in issue #584 (closed)

We're not currently using prefixed branches, closing this as invalid for the moment.

closed

added DeliveryP1 label

removed priority1 label