Sidekiq callback instead of internals for graceful shutdown

Happens at the same time as the old code: https://github.com/sidekiq/sidekiq/blob/73c150d0430a8394cadb5cd49218895b113613a0/lib/sidekiq/launcher.rb#L47-L54 but this is less brittle.

Sidekiq::CLI is only defined when Sidekiq runs as a standalone process. Since 7.0 it can also run embedded in the web server process, and using this hook allows Iteration to be compatible with mode of operation. The same events are fired here: https://github.com/sidekiq/sidekiq/blob/73c150d0430a8394cadb5cd49218895b113613a0/lib/sidekiq/embedded.rb#L24-L30

lib/job-iteration/integrations/sidekiq.rb

@@ -4,11 +4,21 @@
 
 module JobIteration
   module Integrations # @private
-    JobIteration.interruption_adapter = -> do
-      if defined?(Sidekiq::CLI) && Sidekiq::CLI.instance
-        Sidekiq::CLI.instance.launcher.stopping?
-      else
-        false
+    module Sidekiq
+      class << self
+        attr_accessor :stopping
+
+        def call
+          stopping
+        end
+      end
+    end
+
+    JobIteration.interruption_adapter = JobIteration::Integrations::Sidekiq
+
+    ::Sidekiq.configure_server do |config|
+      config.on(:quiet) do
+        JobIteration::Integrations::Sidekiq.stopping = true
       end
     end
   end