Remove `updatedAt` field from Open Payments RS responses

### Context

Since 

[https://github.com/interledger/open-payments/issues/532](https://github.com/interledger/open-payments/issues/532) ([OPE-162](https://linear.app/interledger/issue/OPE-162/remove-updatedat-from-spec))

was completed in the Open Payments, we should update `backend` 's resource server to remove `updatedAt` field from the incoming and outgoing payment responses. 