Improve flutter_gen/l10n migration docs

[divan]

Page URL

https://docs.flutter.dev/release/breaking-changes/flutter-generate-i10n-source/

Page source

https://github.com/flutter/website/tree/main/src/content/release/breaking-changes/flutter-generate-i10n-source.md

Describe the problem

Documentation about transition/migration from old flutter_gen way is incomplete. It offers two solutions, but if you apply them to the existing (old) project that uses flutter_gen, the problem still persists.

Expected fix

The missing steps are:

3. Replace:

import 'package:flutter_gen/gen_l10n/app_localizations.dart';

to

import 'l10n/app_localizations.dart';

in your dart files.

Additional context

No response

I would like to fix this problem.

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

[sfshaza2]

@matanlurey, this issue appears to be correct. Can we add a link about updating the location of the import?