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.

modules/mono/glue/cs_files/Color.cs

@@ -258,11 +258,6 @@ namespace Godot
             return res;
         }
 
-        public float Gray()
-        {
-            return (r + g + b) / 3.0f;
-        }
-
         public Color Inverted()
         {
             return new Color(