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

Docs are in gitlab!30542 (merged)

We can remove the mess in gitlab-com/gl-infra/scalability#240