Make time_zone_aware_attributes a class attribute

Followup: rails#42442

Author: byroot

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -63,8 +63,7 @@ def map_avoiding_infinite_recursion(value)
       extend ActiveSupport::Concern
 
       included do
-        mattr_accessor :time_zone_aware_attributes, instance_writer: false, default: false
-
+        class_attribute :time_zone_aware_attributes, instance_writer: false, default: false
         class_attribute :skip_time_zone_conversion_for_attributes, instance_writer: false, default: []
         class_attribute :time_zone_aware_types, instance_writer: false, default: [ :datetime, :time ]
       end