Move singleton require.

ActiveSupport::Deprecation used to `include Singleton` which is why the require is here, but it was removed in 9812641 (June 5 2023). Leaving the require seems to have been an oversight.

However, module ActiveJob::Serializers::ObjectSerializer does require Singleton and was relying on AS:Deprecation to require it. This PR just moves the require to the place it's actually used.

Author: wjessop

activejob/lib/active_job/serializers/object_serializer.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "singleton"
+
 module ActiveJob
   module Serializers
     # Base class for serializing and deserializing custom objects.

activesupport/lib/active_support/deprecation.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "singleton"
-
 module ActiveSupport
   # = Active Support \Deprecation
   #