Core/AI Prevent bosses respawn when BossState is set to DONE (#17616)

(cherry picked from commit f826ecb4b2e8cc1fa54419947dff041a580d9c03)

src/server/game/AI/ScriptedAI/ScriptedCreature.cpp

@@ -496,6 +496,14 @@ void BossAI::_EnterCombat()
     ScheduleTasks();
 }
 
+bool BossAI::CanRespawn()
+{
+    if (instance && instance->GetBossState(_bossId) == DONE)
+        return false;
+
+    return true;
+}
+
 void BossAI::TeleportCheaters()
 {
     float x, y, z;