The point is that `RayCast`s are checked against objects' `collision_layer`(s), but they themselves are considered no to _belong_ to any layer. Therefore, the correct name for their property is `collision_mask`, rather than `collision_layer`.
Only renaming is needed since the behavior was already the right one, only that it wasn't matching what users would expect from the name and description of the property.
Fixes#7589, where it's also discussed.
