Predictive back migration guide confirmation dialog example (#9244)

A user requested this example be added to the migration guide [on
Discord](https://discord.com/channels/608014603317936148/969301283825659914/1140422179511603290).
I think it's a reasonable request.

Follow up on: #8952
Framework PR: flutter/flutter#132249

---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>

src/release/breaking-changes/android-predictive-back.md

@@ -270,9 +270,60 @@ if (_route.popDisposition == RoutePopDisposition.doNotPop) {
 }
 ```
 
+### Migrating a back confirmation dialog
+
+`WillPopScope` was sometimes used to show a confirmation dialog when
+a back gesture was received. 
+This can still be done with `PopScope` in a similar pattern.
+
+Code before migration:
+
+```dart
+WillPopScope(
+  onWillPop: () async {
+    final bool? shouldPop = await _showBackDialog();
+    return shouldPop ?? false;
+  },
+  child: child,
+)
+```
+
+Code after migration:
+
+```dart
+return PopScope(
+  canPop: false,
+  onPopInvoked: (bool didPop) async {
+    if (didPop) {
+      return;
+    }
+    final NavigatorState navigator = Navigator.of(context);
+    final bool? shouldPop = await _showBackDialog();
+    if (shouldPop ?? false) {
+      navigator.pop();
+    }
+  },
+  child: child,
+)
+```
+
+### Supporting predictive back
+
+  1. Run Android 33 or above.
+  1. Enable the feature flag for predictive back on
+     the device under "Developer options".
+     This will be unnecessary on future versions of Android.
+  1. Set `android:enableOnBackInvokedCallback="true"` in
+     `android/app/src/main/AndroidManifest.xml`. If needed, refer to
+     [Android's full guide](https://developer.android.com/guide/navigation/custom-back/predictive-back-gesture)
+     for migrating Android apps to support predictive back.
+  1. Make sure you're using version `3.14.0-7.0.pre` of Flutter or greater.
+  1. Run the app and perform a back gesture (swipe from the left side of
+     the screen).
+
 ## Timeline
 
-Landed in version: 3.14.0-0.0.pre<br>
+Landed in version: 3.14.0-7.0.pre<br>
 In stable release: not yet
 
 ## References