Use get_node_light for sunlight in 5.3

Avoids servers running 5.3 crashing.

mods/ENTITIES/mcl_mobs/api.lua

@@ -1047,7 +1047,10 @@ local do_env_damage = function(self)
 		end
 	end
 
-	local sunlight = minetest.get_natural_light(pos, self.time_of_day)
+	-- Use get_node_light for Minetest version 5.3 where get_natural_light
+	-- does not exist yet.
+	local get_light = minetest.get_natural_light or minetest.get_node_light
+	local sunlight = get_light(pos, self.time_of_day)
 
 	-- bright light harms mob
 	if self.light_damage ~= 0 and (sunlight or 0) > 12 then