Add runbook on resolving sync_remotes failures
At the end of the last critical security patch release, we encountered troubles syncing gitlab canonical and security repositories. This runbook is in hopes to make the sync error troubleshooting process smoother.
Slack discussion: https://gitlab.slack.com/archives/C0139MAV672/p1684907440345189?thread_ts=1684862762.912809&cid=C0139MAV672
When this is part of the runbooks, it should be mentioned in the security patch template.