* Migrates to `package:web` https://github.com/flutter/flutter/issues/139749
* Updates `HttpRequestFactory.request` to use `package:http` `BrowserClient`
* Updates `ìndex.html` in the example to use `flutter_bootstrap.js`
* Updates minimum dart sdk requirement to `^3.3.0`

Would appreciate help with completing the mock tests if in case it does not work.
(I am somehow stuck with 'loading...' when attempting to test with mockito with --platform chrome)

Integration tests from what i was able to test passes.

Migrated to using BrowserClient for web due to issues creating mock tests with `XMLHttpRequest` which is returned from `package:web`'s `HttpRequest.request` following error:

`Bad state: Interface type 'XMLHttpRequest' which is nether an enum, nor a class, nor a mixin. This case is unknown, please report a bug.`

Co-authored-by: David Iglesias <[email protected]>
Co-authored-by: Navaron Bracke <[email protected]>