Run Sidekiq Cluster supervision on Core

Now that the Sidekiq Cluster script is available in Core
installations and soon will be used by GDK and Omnibus,
we need to also run the process supervision to terminate
a Cluster process if its parent was SIGKILL'd.

config/initializers/sidekiq_cluster.rb

 # frozen_string_literal: true
 
-if ENV['ENABLE_SIDEKIQ_CLUSTER'] && Gitlab.ee?
+if ENV['ENABLE_SIDEKIQ_CLUSTER']
   Thread.new do
     Thread.current.abort_on_exception = true