update signature to use generic type

Author: Phil Varner

src/stapi_fastapi/backends/root_backend.py

@@ -34,7 +34,7 @@ async def get_order(self, order_id: str, request: Request) -> ResultE[Maybe[Orde
 
     async def get_order_statuses(
         self, order_id: str, request: Request
-    ) -> ResultE[list[OrderStatus]]:
+    ) -> ResultE[list[U]]:
         """
         Get statuses for order with `order_id`.