Move queue selector to separate snippet

What does this MR do?

The sidekiq['queue_selector'] setting is deprecated and removed as of GitLab 17.0. If a user follows the current instructions from GitLab 17.0+, it will cause an error on reconfigure:

Running handlers:
[2024-07-04T10:21:46+12:00] ERROR: Running exception handlers
There was an error running gitlab-ctl reconfigure:

Removed configurations found in gitlab.rb. Aborting reconfigure.


Removals:
* sidekiq['queue_selector'] has been deprecated since 15.9 and was removed in 17.0. Starting with GitLab 17.0, running Sidekiq with queue selector (`sidekiq['queue_selector'] = true`) will be removed. We recommend to follow the steps at https://docs.gitlab.com/ee/administration/sidekiq/extra_sidekiq_processes.html#start-multiple-processes, to run Sidekiq with multiple processes while listening to all queues.

Let's make this part optional depending on the user's GitLab version.

Related issues

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