refactor: improve stone cooldown update method

src/main/java/uulm/teamname/marvelous/gamelibrary/gamelogic/StoneCooldownManager.java

@@ -41,7 +41,7 @@ public class StoneCooldownManager {
      * Decreases all cooldowns by one according to a round having passed.
      */
     public void update() {
-        cooldowns.replaceAll((s, v) -> Math.max(0, cooldowns.getOrDefault(s, 0) - 1));
+        cooldowns.replaceAll((s, v) -> Math.max(0, v - 1));
     }
 
     /**