`WidgetsApp.debugShowWidgetInspectorOverride` was replaced with ` WidgetsBinding.instance.debugShowWidgetInspectorOverrideNotifier` in https://github.com/flutter/flutter/pull/144029.

The old API was removed, not deprecated.

It is used by some [open-source projects](https://github.com/search?q=WidgetsApp.debugShowWidgetInspectorOverride&type=code), thus I'm making the effort of bringing the API back as deprecated.

Fixes https://github.com/flutter/flutter/issues/145333