Update localization page to account for deprecated flutter_gen

[jeffscaturro-aka]

Page URL

https://docs.flutter.dev/ui/accessibility-and-internationalization/internationalization#adding-your-own-localized-messages

Page source

https://github.com/flutter/website/tree/main/src/content/ui/accessibility-and-internationalization/internationalization.md

Describe the problem

We are now warning users that flutter_gen is a synthetic package and have provided migration documentation for it.

However, as part of that effort it seems that updating the Flutter documentation was missed, and encourages now bad practices we are actively trying to migrate away from.

Expected fix

Documentation should always be updated to account for current best practices, especially when we're actively working on pivoting to a new best practice.

Specifically updating the following section with now best practices: Adding your own localized message

Additional context

No response

I would like to fix this problem.

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