Merge pull request #20236 from smoogipoo/fix-countdown-serialisation

Fix countdown serialisation

osu.Game/Online/Multiplayer/MultiplayerCountdown.cs

@@ -33,6 +33,7 @@ namespace osu.Game.Online.Multiplayer
         /// <summary>
         /// Whether only a single instance of this <see cref="MultiplayerCountdown"/> type may be active at any one time.
         /// </summary>
+        [IgnoreMember]
         public virtual bool IsExclusive => true;
     }
 }