Update recipe to use Firebase Test Lab on 'An introduction to integration testing' page

[SpiegelSoft]

Page URL

https://flutter.dev/docs/cookbook/testing/integration/introduction.html

Page source

https://github.com/flutter/website/tree/master/src/docs/cookbook/testing/integration/introduction.md

Describe the problem

This describes the legacy integration testing procedure, which is not compatible with Flutter Test Lab.

Expected fix

Please upgrade the documentation to show the newer version of integration testing

Additional context

No response

[TahaTesser]

Hi @SpiegelSoft This cookbook is outdated, on top of the page you can see a note, please follow the link, there you can find update instructions and steps to run your test on Firebase Test Lab

~~I can work on updating this cookbook~~

[TahaTesser]

@atsansone Unfortunately, i'm swamped in Flutter issues in the flutter/flutter repo, working on couple of big projects :) Feel free to assign anyone else.