Fix background colour for dark theme

The default colour for Rectangle is white. So this Rectangle, not defining a colour, became white. Instead we should use an Item, which cannot have a background colour and thus becomes transparent, making the background colour the same as the button below, which was intended. It's also slightly faster to render.

Contributes to issue CURA-6033.

resources/qml/Menus/ConfigurationMenu/ConfigurationItem.qml

@@ -73,7 +73,7 @@ Button
             }
 
             // Unknown material
-            Rectangle
+            Item
             {
                 id: unknownMaterial
                 height: unknownMaterialMessage.height + UM.Theme.getSize("thin_margin").width / 2