fix: adapt scrollable.dart to upstream SemanticsNode changes (#387)

This PR https://github.com/flutter/flutter/pull/137304 makes SemanticsNode.isMergedIntoParent readonly, which makes the customized scrollable.dart incompatible.

@@ -866,10 +866,7 @@ class _RenderScrollSemantics extends RenderProxyBox {
       return;
     }
 
-    _innerNode ??= SemanticsNode(showOnScreen: showOnScreen);
-    _innerNode!
-      ..isMergedIntoParent = node.isPartOfNodeMerging
-      ..rect = node.rect;
+    (_innerNode ??= SemanticsNode(showOnScreen: showOnScreen)).rect = node.rect;
 
     int? firstVisibleIndex;
     final List<SemanticsNode> excluded = <SemanticsNode>[_innerNode!];