docs: add UISceneDelegate migration warning to iOS platform views

[lamek]

Added a version note advising that didInitializeImplicitFlutterEngine is required when using the default UIScene lifecycle on iOS 13+

_Issues fixed by this PR (if any): #13087

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• 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 pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request updates the iOS platform views documentation to include a version note regarding the default adoption of the UIScene lifecycle starting from Flutter 3.41.0. It details the requirement to implement FlutterImplicitEngineDelegate and register platform views inside the appropriate initialization method for both Swift and Objective-C, and adds a reference link to the UISceneDelegate adoption guide. There are no review comments, so I have no feedback to provide.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit c2b12b2):

https://flutter-docs-prod--docs-pr13668-fix-13087-7qwrf47x.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit c2b12b2):

https://flutter-dev-230821--www-pr13668-fix-13087-mnpn45og.web.app