Previously we were determining the gitlab_schema from the `table_name`.
This is causing problems as we're migrating lots of tables to
`gitlab_main_cell`. But the `gitlab_schema` must exactly match whatever
schema was used to queue the batched background migration (see
https://docs.gitlab.com/ee/development/database/batched_background_migrations.html#finalize-a-batched-background-migration
) . As such we've extracted this value from the migration record in the
database which should always be the expected value.