Update Sidekiq system check when running with a cluster becomes the only supported mode
Summary
Running without a cluster is deprecated and will be removed in %14.0: https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/install/installation.md#using-sidekiq-instead-of-sidekiq-cluster
As a follow-up to !55530 (merged), we can simplify the system check to always require 1 cluster process and 1+ workers.
Related to gitlab-com/gl-infra&198 (closed).
Involved components
lib/system_check/sidekiq_check.rb
Edited by Arturo Herrero