Backend: Remove `ci_build_finished_worker_namespace_changed` feature flag and migrate queue

We have to remove the feature flag and migrate the queue in a major release for ci_build_finished_worker_namespace_changed flag.

This is similar to renaming a queue which involves a post deploy migration.

Yes

Self-managed users

mention your stage's stable counterparts on this issue. For example, Customer Support, Customer Success (Technical Account Manager), Product Marketing Manager.
- @supadhyaya, @calebcooper, @jrreid @dzalbo, @jreporter - This is a breaking change for SM users in %15.0 that will require a post deploy migration to rename the queue.
- To see who the stable counterparts are for a product team visit product categories
  - If there is no stable counterpart listed for Sales/CS please mention @timtams
  - If there is no stable counterpart listed for Support please mention @gitlab-com/support/managers
  - If there is no stable counterpart listed for Marketing please mention @cfoster3
mention your GPM so that they are aware of planned deprecations. The goal is to have reviews happen at least two releases before the final removal of the feature or introduction of a breaking change.