pba: Prioritize gateway's contact fields in update operations

Change the priority of which contact is used when sending update
notifications, instead of using the contacts present within the
ContactInfo list both AdministrativeContact and TechnicalContact have
the priority.

pkg/packetbrokeragent/grpc_gspba.go

@@ -158,7 +158,19 @@ func (s *gsPbaServer) UpdateGateway(ctx context.Context, req *ttnpb.UpdatePacket
 	}
 
 	if ttnpb.HasAnyField(req.FieldMask.GetPaths(), "contact_info") {
+		reqAdminContact := &packetbroker.ContactInfo{Email: req.Gateway.GetAdministrativeContact().GetUserIds().Email}
+		reqTechContact := &packetbroker.ContactInfo{Email: req.Gateway.GetTechnicalContact().GetUserIds().Email}
 		adminContact, techContact := toPBContactInfo(req.Gateway.GetContactInfo())
+
+		// Prioritize administrative and technical contacts attached to the gateway instead of the ones present in
+		// ContactInfo slice.
+		if req.Gateway.AdministrativeContact != nil {
+			adminContact = reqAdminContact
+		}
+		if req.Gateway.TechnicalContact != nil {
+			adminContact = reqTechContact
+		}
+
 		updateReq.AdministrativeContact = &packetbroker.ContactInfoValue{
 			Value: adminContact,
 		}