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.