### Description

This PR is to improve `DropdownMenu` sample code. By default, `requestFocusOnTap` is false on mobile platforms. When users run API sample code on mobile platforms, they can not edit the text field and think it is a bug. Although it is detailed at https://api.flutter.dev/flutter/material/DropdownMenu/requestFocusOnTap.html, users often do not pay attention to it.

### Related issue

Fixes https://github.com/flutter/flutter/issues/127672