Update internationalization documents and gen_l10n example.

[amir14a]

Update internationalization documents and gen_l10n example to align with deprecation of synthetic packages.
From flutter 3.32.0 breaking changes, localized messages are generated into source, not a synthetic package.
This PR should be merged in the future stable release.
Issues fixed by this PR (if any):

• Update localization page to account for deprecated flutter_gen #11765

Relative links:

• https://groups.google.com/g/flutter-announce/c/ZWtR2h6QAZk
• https://flutter.dev/to/flutter-gen-deprecation
• Deprecate and remove synthetic package:flutter_gen flutter#102983
• Add a warning/additional handlers for parsingsynthetic-package. flutter#157934

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[amir14a]

@sfshaza2 @antfitch @parlough