enable Material 3 on platform_design (#1624)

platform_design/lib/main.dart

@@ -25,6 +25,7 @@ class MyAdaptingApp extends StatelessWidget {
       theme: ThemeData(
         // Use the green theme for Material widgets.
         primarySwatch: Colors.green,
+        useMaterial3: true,
       ),
       darkTheme: ThemeData.dark(),
       builder: (context, child) {