Revert board milestone changes from `nil` to `-1`
Problem to solve
Some of the boards were modified with a database migration that needs to be reverted, but it can't just be reversed in the obvious way.
Related reversion MR:
- Original MR (EE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14586
- Original MR (CE port): gitlab-ce!30613
- Reversion issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/65676
Further details
What does success look like, and how can we measure that?
The boards that had -1
before this migration was run have -1
restored as their milestone_id