In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37835 we've
introduced the `relative_position` column and are setting it for every
newly created design.

The post migration queries all issues having designs, and bulk-queues
background migrations for them.

The feature flag can be enabled after all migrations have completed.
If it's enabled before then, the ordering will be temporarily incorrect,
but otherwise this won't cause any harm.

This means:

- We don't need to worry about newly created issues, or issues that
  previously didn't have designs but do now, since they will already
  have a relative position set.

- We don't need to do any clean up later.