Update MenuAnchor API examples (#156404)

## Description

Cleanup MenuAnchor API examples:
- Remove usage of useMaterial3
- fix some formatting issues

examples/api/lib/material/menu_anchor/checkbox_menu_button.0.dart

@@ -101,9 +101,8 @@ class MenuApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      theme: ThemeData(useMaterial3: true),
-      home: const Scaffold(body: SafeArea(child: MyCheckboxMenu(message: kMessage))),
+    return const MaterialApp(
+      home: Scaffold(body: SafeArea(child: MyCheckboxMenu(message: kMessage))),
     );
   }
 }

examples/api/lib/material/menu_anchor/menu_accelerator_label.0.dart

@@ -103,7 +103,6 @@ class MenuAcceleratorApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
-      theme: ThemeData(useMaterial3: true),
       home: Shortcuts(
         shortcuts: <ShortcutActivator, Intent>{
           const SingleActivator(LogicalKeyboardKey.keyT, control: true): VoidCallbackIntent(() {

examples/api/lib/material/menu_anchor/menu_anchor.0.dart

@@ -202,9 +202,8 @@ class MenuApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      theme: ThemeData(useMaterial3: true),
-      home: const Scaffold(body: SafeArea(child: MyCascadingMenu(message: kMessage))),
+    return const MaterialApp(
+      home: Scaffold(body: SafeArea(child: MyCascadingMenu(message: kMessage))),
     );
   }
 }

examples/api/lib/material/menu_anchor/menu_anchor.2.dart

@@ -6,7 +6,6 @@ import 'package:flutter/material.dart';
 
 /// Flutter code sample for [MenuAnchor].
 
-
 void main() => runApp(const MenuAnchorApp());
 
 // This is the type used by the menu below.
@@ -17,9 +16,8 @@ class MenuAnchorApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      theme: ThemeData(useMaterial3: true),
-      home: const MenuAnchorExample(),
+    return const MaterialApp(
+      home: MenuAnchorExample(),
     );
   }
 }
@@ -43,8 +41,7 @@ class _MenuAnchorExampleState extends State<MenuAnchorExample> {
       ),
       body: Center(
         child: MenuAnchor(
-          builder:
-              (BuildContext context, MenuController controller, Widget? child) {
+          builder: (BuildContext context, MenuController controller, Widget? child) {
             return IconButton(
               onPressed: () {
                 if (controller.isOpen) {
@@ -55,7 +52,8 @@ class _MenuAnchorExampleState extends State<MenuAnchorExample> {
               },
               icon: const Icon(Icons.more_horiz),
               tooltip: 'Show menu',
-            );},
+            );
+          },
           menuChildren: List<MenuItemButton>.generate(
             3,
             (int index) => MenuItemButton(

examples/api/test/material/menu_anchor/menu_bar.0_test.dart

@@ -51,6 +51,7 @@ void main() {
     expect(find.text(example.MenuBarApp.kMessage), findsOneWidget);
     expect(find.text('Last Selected: Show Message'), findsOneWidget);
   });
+
   testWidgets('Shortcuts work', (WidgetTester tester) async {
     await tester.pumpWidget(
       const example.MenuBarApp(),