Enable sidekiq-cluster by default
This is the next step in having a single way of running Sidekiq: the Sidekiq config now delegates to sidekiq-cluster by default.
We emit warnings when
sidekiq['cluster'] is disabled
We emit a warning when
sidekiq_cluster['enable'] is set manually, and we fall back to starting Sidekiq with the old configuration.
Related to gitlab-com/gl-infra/scalability#239 (closed)
Docs are in gitlab!30542 (merged)
We can remove the mess in gitlab-com/gl-infra/scalability#240