sticky_header [nfc]: Use new SlottedMultiChildRenderObjectWidget base…

… class

This was introduced recently upstream, deprecating the old mixin
SlottedMultiChildRenderObjectWidgetMixin.

This change follows up on 2f0f469 (from #103), which handled the
mandatory part of the same upstream API change.

lib/widgets/sticky_header.dart

@@ -210,9 +210,8 @@ class RenderSliverStickyHeaderList extends RenderSliverList {
 
 enum StickyHeaderSlot { header, content }
 
-class StickyHeader extends RenderObjectWidget
-    with SlottedMultiChildRenderObjectWidgetMixin<StickyHeaderSlot, RenderBox> {
-  StickyHeader(
+class StickyHeader extends SlottedMultiChildRenderObjectWidget<StickyHeaderSlot, RenderBox> {
+  const StickyHeader(
       {super.key,
       this.direction = AxisDirection.down,
       this.header,