Fix a migration that adds merge_requests_ff_only_enabled column to MR table

What does this MR do?

I found that we mistakenly added merge_requests_ff_only_enabled column to db/schema.rb in 8.2.* releases and then removed in 8.3.* and higher. So if you installed GitLab since 8.2 you will be affected by this issue.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/39382#note_45716103

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39382 and https://gitlab.com/gitlab-org/gitlab-ce/issues/39357