Update Impeller docs for Q4 release

[sfshaza2]

Page URL

https://docs.flutter.dev/perf/impeller

Page source

No response

Describe the problem

As of the Q4 stable release, Impeller is the default for Android.

Expected fix

Update this page and anywhere else it's mentioned on the site.

Additional context

No response

I would like to fix this problem.

• [ ] I will try and fix this problem on docs.flutter.dev.