backends: Update cursor names to match updated metaphors
adwaita-icon-theme updated its cursor metaphors and changed all DnD cursors to use arrows instead of hands, except for the grab related ones. Mutter was using "grabbing" as default DnD cursor, which now does not match the other DnD cursors ("copy" and "no-drop") anymore. Change this to the "default" cursor.
Additionally, because the "no-drop" cursor now puts a stronger emphasis on the crossed out symbol also prefer "default" for META_CURSOR_DND_IN_DRAG and only use "no-drop" for things that explicitly don't accept a drop.
Related: adwaita-icon-theme!63 (merged)