Update on the background migrations:
All except one background migrations finished successfully, leaving only
"BackfillIntegrationsTypeNew: integrations
<https://git.ad-sys.de/admin/background_migrations/15>" stuck at failed.
This one is exactly the migration mentioned in [1], so I guess this one
might cause trouble in general
I then tried to migrate manually with `gitlab-rake db:migrate:redo
VERSION=20210727113447`. This actually worked, however, I still have the
Background Migration job in the admin page. So I guess I simply set the
job's state on success as I don't experience any issues.
