Changed the DataGrid mouse wheel delta from 2 to 3 lines (48 pixels -> 72 pixels)

The default Windows scroll is 3 lines, which requires a lot less scrolling than the current default.

This value is currently hardcoded and can't be changed elsewhere.

src/Avalonia.Controls.DataGrid/DataGrid.cs

@@ -65,7 +65,7 @@ namespace Avalonia.Controls
         private const double DATAGRID_minimumColumnHeaderHeight = 4;
         internal const double DATAGRID_maximumStarColumnWidth = 10000;
         internal const double DATAGRID_minimumStarColumnWidth = 0.001;
-        private const double DATAGRID_mouseWheelDelta = 48.0;
+        private const double DATAGRID_mouseWheelDelta = 72.0;
         private const double DATAGRID_maxHeadersThickness = 32768;
 
         private const double DATAGRID_defaultRowHeight = 22;