Adjust flutter_maps_firestore's lint rules (#821)

flutter_maps_firestore/analysis_options.yaml

@@ -17,4 +17,3 @@ linter:
     test_types_in_equals: true
     throw_in_finally: true
     unnecessary_statements: true
-    use_key_in_widget_constructors: false

flutter_maps_firestore/lib/main.dart

@@ -22,10 +22,12 @@ final _placesApiClient = GoogleMapsPlaces(apiKey: googleMapsApiKey);
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   await Firebase.initializeApp();
-  runApp(App());
+  runApp(const App());
 }
 
 class App extends StatelessWidget {
+  const App({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
@@ -40,8 +42,7 @@ class App extends StatelessWidget {
 }
 
 class HomePage extends StatefulWidget {
-  const HomePage({required this.title});
-
+  const HomePage({required this.title, Key? key}) : super(key: key);
   final String title;
 
   @override