This is a corrective migration to keep the instance column and its
index. Upgrade from 12.7 to 12.9 removes the instance column as it was
first added in the normal migration and then removed in the post
migration.

12.8 removed the instance column in a post deployment migration
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24885
12.9 added the instance column in a normal migration
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25714