fix: replace scrollableBuilder with builder in CupertinoSheetRoute for simplified sheet implementation

cupertino_gallery/lib/widgets/sheet_page.dart

@@ -13,9 +13,7 @@ class SheetPage extends StatelessWidget {
           onPressed: () {
             Navigator.of(context).push(
               CupertinoSheetRoute<void>(
-                scrollableBuilder:
-                    (BuildContext context, ScrollController controller) {
-                      Widget widgetBuilder(BuildContext context) {
+                builder: (BuildContext context) {
                   return CupertinoPageScaffold(
                     navigationBar: CupertinoNavigationBar(
                       middle: const Text('Sheet'),
@@ -28,9 +26,6 @@ class SheetPage extends StatelessWidget {
                     ),
                     child: const Center(child: Text('This is a sheet')),
                   );
-                      }
-
-                      return widgetBuilder(context);
                 },
               ),
             );