Fortunately, there didn't seem to be any legitimate bug that I needed
to fix here. The only that needed to be fixed were because:

- Migrations older than 2019 were removed with 13.1 (02ea9bc7), so the
  specs that were relying on a 2018* schema wouldn't work anymore
- The ghost user type was refactored by 3150ee76 so
  `let(:user_type) { 'ghost' }` needed to be changed to
  `let(:user_type) { HasUserType::USER_TYPES[:ghost] }`
- Some post migrations to steal background migrations were left over
  even though the background migration was removed in 13.1. Since
  GitLab needs to be updated to 13.0 first, before being upgrade to
  13.1, it's safe to remove these post migrations now.
- The snippets_size column was added on `20200622095419` but
  `db/post_migrate/20190527194900_schedule_calculate_wiki_sizes.rb`
  might try to update project statistics before the snippets_size
  column has been created.
Signed-off-by: Rémy Coutable <remy@rymai.me>