Core/Realms: Realmlist refactors

* Removed global realm variable from World and use RealmList everywhere
* Match auth build key with client version
* Restored allowedSecurityLevel checks for realmlist packet building
* Restored updating population field
Port From (https://github.com/TrinityCore/TrinityCore/commit/c4b710446d62c95eb8124175203fa5f394912594)

Source/Game/Handlers/HotfixHandler.cs

@@ -49,7 +49,7 @@ namespace Game
         void SendAvailableHotfixes()
         {
             AvailableHotfixes availableHotfixes = new();
-            availableHotfixes.VirtualRealmAddress = Global.WorldMgr.GetRealmId().GetAddress();
+            availableHotfixes.VirtualRealmAddress = Global.WorldMgr.GetVirtualRealmAddress();
 
             foreach (var (_, push) in Global.DB2Mgr.GetHotfixData())
             {