Mark queue-selector option to sidekiq-cluster as no longer experimental

Following #45 (closed) and gitlab-org/gitlab!18877 (merged), we have a new option for sidekiq-cluster: --experimental-queue-selector.

This is marked as experimental, but shouldn't stay that way forever. We should target 13.0 (2020-05-22) to un-mark this as such, but if it's not ready by that point, we shouldn't force it. To do that, we'd want to:

• Rails
  • Support both --experimental-queue-selector and --queue-selector in the CLI.
  • Update the documentation in https://docs.gitlab.com/ee/administration/operations/extra_sidekiq_processes.html to match this.
• Omnibus
  • Support both experimental_queue_selector and queue_selector in gitlab.rb.
  • Add this to the default gitlab.rb template, as this was left out of gitlab-org/omnibus-gitlab!3920 (merged) because it's experimental 🙂
• Charts
  • Support both queueSelector and experimentalQueueSelector (but not both).
  • Update the documentation to match.
  • Update the CNG build images to support either (no need to check for both here).
• Release post
  • Announce this change in the release post.
• Configuration
  • Update our chef (https://ops.gitlab.net/gitlab-cookbooks/chef-repo/-/merge_requests/4606) and k8s config to use the new option.
• Create a follow-up issue to remove support for the 'experimental' variants of these arguments / config options (in 1 and 2) in the next yearly release. So if we do this in 13.0, the next issue should target 14.0.

All of this assumes that this experimental syntax doesn't undergo significant changes between now and then; if it does, then we should obviously rethink 🙂