Scripts/Ulduar: Fix Flame Leviathan instance lock

Fix Flame Leviathan instance lock not being granted to players because they were ejected from the vehicles as soon as boss died.
Players are now ejected some seconds later.
Close https://github.com/TrinityCore/TrinityCore/issues/15005

src/server/scripts/Northrend/Ulduar/Ulduar/instance_ulduar.cpp

@@ -690,22 +690,7 @@ class instance_ulduar : public InstanceMapScript
                 {
                     case BOSS_LEVIATHAN:
                         if (state == DONE)
-                        {
+                            _events.ScheduleEvent(EVENT_DESPAWN_LEVIATHAN_VEHICLES, 5 * IN_MILLISECONDS);
-                            // Eject all players from vehicles and make them untargetable.
-                            // They will be despawned after a while
-                            for (auto const& vehicleGuid : LeviathanVehicleGUIDs)
-                            {
-                                if (Creature* vehicleCreature = instance->GetCreature(vehicleGuid))
-                                {
-                                    if (Vehicle* vehicle = vehicleCreature->GetVehicleKit())
-                                    {
-                                        vehicle->RemoveAllPassengers();
-                                        vehicleCreature->SetFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_NOT_SELECTABLE);
-                                        vehicleCreature->DespawnOrUnsummon(5 * MINUTE * IN_MILLISECONDS);
-                                    }
-                                }
-                            }
-                        }
                         break;
                     case BOSS_IGNIS:
                     case BOSS_RAZORSCALE:
@@ -1164,6 +1149,22 @@ class instance_ulduar : public InstanceMapScript
                                     algalon->AI()->DoAction(EVENT_DESPAWN_ALGALON);
                             }
                             break;
+                        case EVENT_DESPAWN_LEVIATHAN_VEHICLES:
+                            // Eject all players from vehicles and make them untargetable.
+                            // They will be despawned after a while
+                            for (auto const& vehicleGuid : LeviathanVehicleGUIDs)
+                            {
+                                if (Creature* vehicleCreature = instance->GetCreature(vehicleGuid))
+                                {
+                                    if (Vehicle* vehicle = vehicleCreature->GetVehicleKit())
+                                    {
+                                        vehicle->RemoveAllPassengers();
+                                        vehicleCreature->SetFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_NOT_SELECTABLE);
+                                        vehicleCreature->DespawnOrUnsummon(5 * MINUTE * IN_MILLISECONDS);
+                                    }
+                                }
+                            }
+                            break;
                     }
                 }
             }

src/server/scripts/Northrend/Ulduar/Ulduar/ulduar.h

@@ -404,6 +404,7 @@ enum UlduarEvents
     EVENT_DESPAWN_ALGALON       = 1,
     EVENT_UPDATE_ALGALON_TIMER  = 2,
     ACTION_INIT_ALGALON         = 6,
+    EVENT_DESPAWN_LEVIATHAN_VEHICLES = 7
 };
 
 enum YoggSaronIllusions