Migrate form_app to Material 3 (#1612)

form_app/lib/main.dart

@@ -88,7 +88,10 @@ class FormApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialApp.router(
       title: 'Form Samples',
-      theme: ThemeData(primarySwatch: Colors.teal),
+      theme: ThemeData(
+        colorSchemeSeed: Colors.teal,
+        useMaterial3: true,
+      ),
       routerConfig: router,
     );
   }

form_app/lib/src/validation.dart

@@ -27,7 +27,6 @@ class _FormValidationDemoState extends State<FormValidationDemo> {
           Padding(
             padding: const EdgeInsets.all(8),
             child: TextButton(
-              style: TextButton.styleFrom(foregroundColor: Colors.white),
               child: const Text('Submit'),
               onPressed: () {
                 // Validate the form by getting the FormState from the GlobalKey