Set rails 6.0 default for `active_storage.replace_on_assign_to_many` (true)
Set rails 6.0 default for `active_storage.replace_on_assign_to_many` (true)

 > Determines whether assigning to a collection of attachments declared with
   `has_many_attached` replaces any existing attachments or appends to them.

We are not using `has_many_attached`, only `has_one_attached` so
should not have any impact.

ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments