Update to fulfillmentHold.heldByApp field from fulfillmentHold.heldBy field

As of the 2025-01 API version, the heldBy string field on the fulfillmentHold GraphQL object will be replaced by the newly introduced heldByApp object field.

This new field provides access to all attributes of the App object, unlike the deprecated heldBy field, which only indicated the app's title.

If you currently query fulfillmentHold.heldBy, you should transition to querying fulfillmentHold.heldByApp.title.