Fix defaults for MR states and merge statuses

What does this MR do?

This ensures that merge_requests.state and merge_requests.merge_status both have a proper default value and NOT NULL constraint on database level. We also make sure to update any bogus rows first, without blowing up the database.

I'll apply these changes manually to GitLab.com so we don't need to wait for the next patch release.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • 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
• Internationalization required/considered

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/40534