…11068)

_Description of what this PR is changing or adding, and why:_
Adds a migration guide telling folks how to opt out of edge-to-edge mode
when their Flutter on Android app targets Android 15+.

This (targeting Android 15+) will happen by default in the next Flutter
stable version, so the described migration steps will be necessary if
folks do not explicitly use edge-to-edge mode in their app. Folks may
also change the Android version they target, so the steps apply if their
target Android 15+ themselves, as well.

_Issues fixed by this PR (if any):_
Part of #10755

_PRs or commits this PR depends on (if any):_
flutter/flutter#153795 (merged)

## Presubmit checklist

- [x] This PR is marked as draft with an explanation if not meant to
land until a future stable release.
- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style) — for example, it
doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [x] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.

---------

Co-authored-by: Shams Zakhour (ignore Sfshaza) <44418985+sfshaza2@users.noreply.github.com>