[go_router] `onPopPage` is deprecated

This deprecation showed up in the routine sweep of flutter/packages; ideally it should be fixed as soon the necessary APIs to do so are on `stable`, to minimize future client disruption.

```
info • lib/src/builder.dart:434:11 • 'onPopPage' is deprecated and shouldn't be used. Use onDidRemovePage instead. This feature was
deprecated after v3.16.0-17.0.pre. Try replacing the use of the deprecated member with the replacement. •
deprecated_member_use
info • test/custom_transition_page_test.dart:55:15 • 'onPopPage' is deprecated and shouldn't be used. Use onDidRemovePage instead. This
feature was deprecated after v3.16.0-17.0.pre. Try replacing the use of the deprecated member with the replacement. •
deprecated_member_use
info • test/custom_transition_page_test.dart:104:15 • 'onPopPage' is deprecated and shouldn't be used. Use onDidRemovePage instead. This
feature was deprecated after v3.16.0-17.0.pre. Try replacing the use of the deprecated member with the replacement. •
deprecated_member_use
info • test/go_router_state_test.dart:209:17 • 'onPopPage' is deprecated and shouldn't be used. Use onDidRemovePage instead. This feature was
deprecated after v3.16.0-17.0.pre. Try replacing the use of the deprecated member with the replacement. •
deprecated_member_use
```

Reference: https://docs.flutter.dev/release/breaking-changes/navigator-and-page-api
It looks like this will be in the next stable release after 3.22?