Warn users at DevTools startup for Flutter version incompatibility

From @jacob314:

> One thing we might want to do is actively fail at startup if you happen to use DevTools with an incompatible version of Flutter. That could happen if someone launched DevTools from one Flutter SDK and connected to a Flutter app from a different Flutter SDK. With this and similar changes we will start mysteriously failing when this happens. Users will assume DevTools is flaky rather than understanding they used an incompatible Flutter unless we warn them.

For this, we will need a way to grab the flutter version that DevTools was built with. We no longer have a flutter-version.txt file, so we will need a different solution to get this information.

首先定位 DevTools 的启动处理逻辑以及连接到 Flutter 应用的代码，然后调查如何在不使用 flutter-version.txt 的情况下获取用于构建 DevTools 的 Flutter 版本。完成标准是：在启动时检测到不兼容的 Flutter SDK，并向用户显示清晰的警告，说明不兼容的原因。

由索引模型根据 Issue 内容生成。