[web][breaking] Change `--web-renderer` default from `auto` to `canvaskit`

[Mouad Debbar]

Issue: https://github.com/flutter/flutter/issues/149826

Summary
The --web-renderer CLI argument's default value is changing from auto to canvaskit.

Background
The CanvasKit renderer has come a long way since the early days of Flutter Web. Many bugs have been fixed, and the size has been reasonably decreased. We now consider the CanvasKit renderer mature enough to be the default renderer for Flutter Web.

Migration guide
We recommend using the canvaskit renderer, and that's why we are making it the default. But if you have a need for the auto renderer, then you need to explicitly pass it as a CLI argument (please give us feedback on why you need auto or html renderers here).

Command before migration:

flutter run -d chrome

Command after migration:

flutter run -d chrome --web-renderer=auto