The deprecated `[RawScrollbar/Scrollbar/CupertinoScrollbar/ScrollbarThemeData].isAlwaysShown` has expired and is removed in thi PR.
Also, `Scrollbar.hoverThickness` is also removed after having expired.

The replacement for `isAlwaysShown` is `thumbVisibility`. This deprecation was introduced in #96957. The name change came from new behaviors like a track that is conditionally visible. Since the original API only referred to the thumb, the name was changed to reflect this intention. This change is supported by dart fix. ✅ 

The replacement for `hoverThickness` is `ScrollbarThemeData.thickness`, which is a MaterialStateProperty that allows for the thickness to respond to multiple states including hover. This deprecation was introduced in #97173. This change is not supported by dart fix. 😞  

Part of https://github.com/flutter/flutter/issues/127042