Core/Group: implement automatic party/raid leader change when the leader has been offline for two minutes

Port From (https://github.com/TrinityCore/TrinityCore/commit/cc141e9bad5dc06a56179f820670bd6944651804)

Source/Game/Groups/GroupManager.cs

@@ -89,6 +89,12 @@ namespace Game.Groups
             return GroupStore.LookupByKey(groupId.GetCounter());
         }
 
+        public void Update(uint diff)
+        {
+            foreach (var group in GroupStore.Values)
+                    group.Update(diff);
+        }
+
         public void AddGroup(Group group)
         {
             GroupStore[group.GetGUID().GetCounter()] = group;
@@ -222,15 +228,6 @@ namespace Game.Groups
             }
         }
 
-        public void Update(uint diff)
-        {
-            foreach (var group in GroupStore.Values)
-            {
-                if (group)
-                    group.Update(diff);
-            }
-        }
-
         Dictionary<ulong, Group> GroupStore = new();
         Dictionary<uint, Group> GroupDbStore = new();
         ulong NextGroupId;