fix(react): add key prop to NotFoundErrorPage route

Avoids:
```
Warning: Each child in a list should have a unique "key" prop.

Check the render method of `App`. See https://fb.me/react-warning-keys for more information.
    in Route
    in App (at src/index.tsx:6)
```

packages/core-api/src/app/App.tsx

@@ -184,7 +184,13 @@ export class PrivateAppImpl implements BackstageApp {
       }
     }
 
-    routes.push(<Route path="/*" element={<NotFoundErrorPage />} />);
+    routes.push(
+      <Route
+        key="not-found-error-page"
+        path="/*"
+        element={<NotFoundErrorPage />}
+      />,
+    );
 
     return routes;
   }