Migrate deprecated requested shipping method names

The order field originalCarrierService and the update input’s shippingMethod argument are deprecated aliases of requestedShippingMethod. Existing names still work. Use the replacement when updating an integration, and do not send conflicting values under the old and new names: those requests are rejected.

The value remains the customer’s original shipping choice. It is distinct from requestedCarrierServiceName, which identifies the carrier service selected to fulfill the order.