Enable Material 3 on provider_counter (#1626)

Co-authored-by: Brett Morgan <brettmorgan@google.com>

provider_counter/lib/main.dart

@@ -69,6 +69,7 @@ class MyApp extends StatelessWidget {
       title: 'Flutter Demo',
       theme: ThemeData(
         primarySwatch: Colors.blue,
+        useMaterial3: true,
       ),
       home: const MyHomePage(),
     );