Deprecate queue selector in favour of worker routing
Once we have done &469, we will be using worker routing everywhere on GitLab.com, and the queue selector nowhere on GitLab.com.
The default for self-managed instances is to use neither, so it would be a good time to start the work of deprecating the queue selector now. This would give us some benefits:
- We can (eventually) remove some code and not have to maintain the queue selector.
- Customers will see reduced Redis CPU usage when they have a lot of Sidekiq threads.
- We can simplify https://gitlab.com/gitlab-org/build/CNG/-/blob/master/gitlab-sidekiq/scripts/process-wrapper to be less baroque.
The trickiest part of this will be around customers who are currently using the queue selector; we probably won't be able to fully remove the queue selector until 16.0.