The introduction of background migrations means some of our requirements
for online upgrades have to be adjusted / clarified.
See https://gitlab.com/gitlab-org/gitlab-ce/issues/35939 for more
information.
Background migrations can be used to perform long running data
migrations without these blocking a deployment procedure.
See MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11854 for
more information.