sticky_header: Pass new second type argument to SlottedContainerRenderObjectMixin et al.

This is required since flutter/flutter#126108 .

The breakage is a reminder that it'd be good to get our repo added
to Flutter's "customer tests" suite.  Because this isn't a common API
to extend, neither that suite nor Google's internal use of Flutter
had anything affected by this change, which mean that Flutter's
"breaking change" policy treated it as a non-breaking change, the
same as an internal refactoring.

In addition to these required changes, there's a deprecation
of SlottedMultiChildRenderObjectWidgetMixin in favor of an abstract
base class.  We can take care of that in a separate commit.

[greg: wrote new commit message]

Author: samwightt

lib/widgets/sticky_header.dart

@@ -211,7 +211,7 @@ class RenderSliverStickyHeaderList extends RenderSliverList {
 enum StickyHeaderSlot { header, content }
 
 class StickyHeader extends RenderObjectWidget
-    with SlottedMultiChildRenderObjectWidgetMixin<StickyHeaderSlot> {
+    with SlottedMultiChildRenderObjectWidgetMixin<StickyHeaderSlot, RenderBox> {
   StickyHeader(
       {super.key,
       this.direction = AxisDirection.down,
@@ -236,14 +236,14 @@ class StickyHeader extends RenderObjectWidget
   }
 
   @override
-  SlottedContainerRenderObjectMixin<StickyHeaderSlot> createRenderObject(
+  SlottedContainerRenderObjectMixin<StickyHeaderSlot, RenderBox> createRenderObject(
       BuildContext context) {
     return RenderStickyHeader(direction: direction);
   }
 }
 
 class RenderStickyHeader extends RenderBox
-    with SlottedContainerRenderObjectMixin<StickyHeaderSlot> {
+    with SlottedContainerRenderObjectMixin<StickyHeaderSlot, RenderBox> {
   RenderStickyHeader({required AxisDirection direction})
       : _direction = direction;