Merge branch 'fix-namespace-deletion' into 'master'

Fix bug where destroying a namespace would not always destroy projects
    
There is a race condition in DestroyGroupService now that projects are deleted asynchronously:
    
1. User attempts to delete group
2. DestroyGroupService iterates through all projects and schedules a Sidekiq job to delete each Project
3. DestroyGroupService destroys the Group, leaving all its projects without a namespace
4. Projects::DestroyService runs later but the can?(current_user,
    :remove_project) is `false` because the user no longer has permission to
    destroy projects with no namespace.
5. This leaves the project in pending_delete state with no namespace/group.
 
Projects without a namespace or group also adds another problem: it's not possible to destroy the container registry tags, since `container_registry_path_with_namespace` is the wrong value.

The fix is to destroy the group asynchronously and run `execute` directly on Projects::DestroyService.
 
Closes #17893

See merge request !4341