Change SliverGeometry.maxScrollObstructionExtent for RenderSliverFloatingPersistentHeader to match docs (#39263)


RenderSliverFloatingPersistentHeader does not pin to the leading edge of the viewport so the value of maxScrollObstructionExtent in it's SliverGeometry should be 0 per the docs at https://api.flutter.dev/flutter/rendering/SliverGeometry/maxScrollObstructionExtent.html.