Core/Scripts: dynamic_spawning follow-up, I had forgotten JustRespawned existed.

- Rename JustRespawned to JustAppeared, which better matches its behavior anyway.
- Properly invoke JustAppeared for new (re-)spawns - fixes #20111.
- Fix Thaddius script to work with dynamic_spawning (mostly unrelated to the above) - Feugen/Stalagg should really be a summon group, but I don't have time to fix that right now.
- Fix default value for DynamicEscortNPC to match worldserver.conf.dist.

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

@@ -145,7 +145,7 @@ void FollowerAI::JustDied(Unit* /*killer*/)
     }
 }
 
-void FollowerAI::JustRespawned()
+void FollowerAI::JustAppeared()
 {
     m_uiFollowState = STATE_FOLLOW_NONE;