Migrate platform_view_swift to Material 3 (#1625)

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

platform_view_swift/lib/main.dart

@@ -20,6 +20,7 @@ class PlatformView extends StatelessWidget {
       title: 'Platform View',
       theme: ThemeData(
         primarySwatch: Colors.grey,
+        useMaterial3: true,
       ),
       home: const HomePage(),
     );