Deprecate incorrect Color::gray()

This average is not a proper approximation of a grayscale value,
get_v() is better suited for that.

If we want a real to_grayscale() conversion, it's somewhat more
involved: https://en.wikipedia.org/wiki/Grayscale

Remove the deprecated Gray() from C# bindings as it conflicts
with new named color constants.

scene/2d/collision_shape_2d.cpp

@@ -119,7 +119,7 @@ void CollisionShape2D::_notification(int p_what) {
 
 			Color draw_col = get_tree()->get_debug_collisions_color();
 			if (disabled) {
-				float g = draw_col.gray();
+				float g = draw_col.get_v();
 				draw_col.r = g;
 				draw_col.g = g;
 				draw_col.b = g;