Use Sidekiq Cluster for running background jobs

This changes the Sidekiq container so that it uses bin/background_jobs which uses Sidekiq cluster.

This is the same way Sidekiq is run in GDK and Omnibus. CNG still calls bundle exec sidekiq directly but it uses arguments from bin/sidekiq-cluster.

This makes it so that we don't depend on config/sidekiq_queues.yml. A problem was found related to this in gitlab!69655 (comment 674330863)