DEV: Route Sidekiq logs to Rails logger (#15817)

Most of our logging goes through Rails.logger, and therefore appears in Logster at `/logs` on a site. The Sidekiq logger was bypassing this and writing directly to STDERR.

Unfortunately it's not possible to do `Sidekiq.logger = Rails.logger` because `Sidekiq#logger=` applies a number of patches to the logger instance, causing our whole logging system to break.

Instead, this commit adds a dedicated Logger instance with no output, which is then patched to forward all messages directly to `Rails.logger`

lib/demon/sidekiq.rb

@@ -27,11 +27,6 @@ class Demon::Sidekiq < ::Demon::Base
 
     puts "Loading Sidekiq in process id #{Process.pid}"
     require 'sidekiq/cli'
-    # CLI will close the logger, if we have one set we can be in big
-    # trouble, if STDOUT is closed in our process all sort of weird
-    # will ensue, resetting the logger ensures it will reinit correctly
-    # parent process is in charge of the file anyway.
-    Sidekiq.logger = Sidekiq::Logger.new(STDERR, level: Logger::WARN)
     cli = Sidekiq::CLI.instance
 
     # Unicorn uses USR1 to indicate that log files have been rotated