Update localization page to account for deprecated flutter_gen

### 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](https://docs.flutter.dev/release/breaking-changes/flutter-generate-i10n-source) 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](https://docs.flutter.dev/ui/accessibility-and-internationalization/internationalization#adding-your-own-localized-messages)

### Additional context

_No response_

### I would like to fix this problem.

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