Use Rails 6 attachments defaults in Active Storage

In Active Storage 5.2 there was an unexpected behavior: assigning a
collection appended records to the existing collection, instead of
replacing them as it's done in Active Record associations.

It doesn't really affect us, though, since we don't use
`has_many_attached` anywhere.

config/initializers/new_framework_defaults_6_0.rb

@@ -28,7 +28,7 @@ Rails.application.config.active_job.return_false_on_aborted_enqueue = true
 
 # When assigning to a collection of attachments declared via `has_many_attached`, replace existing
 # attachments instead of appending. Use #attach to add new attachments without replacing existing ones.
-# Rails.application.config.active_storage.replace_on_assign_to_many = true
+Rails.application.config.active_storage.replace_on_assign_to_many = true
 
 # Use ActionMailer::MailDeliveryJob for sending parameterized and normal mail.
 #