Replace usages of MobObserver and AdminObserver with GameTicker consts (#21814)

Content.Server/GameTicking/GameTicker.GamePreset.cs

@@ -262,7 +262,7 @@ namespace Content.Server.GameTicking
             var xformQuery = GetEntityQuery<TransformComponent>();
             var coords = _transform.GetMoverCoordinates(position, xformQuery);
 
-            var ghost = Spawn("MobObserver", coords);
+            var ghost = Spawn(ObserverPrototypeName, coords);
 
             // Try setting the ghost entity name to either the character name or the player name.
             // If all else fails, it'll default to the default entity prototype name, "observer".

Content.Server/GameTicking/GameTicker.Spawning.cs

@@ -31,6 +31,9 @@ namespace Content.Server.GameTicking
         [ValidatePrototypeId<EntityPrototype>]
         public const string ObserverPrototypeName = "MobObserver";
 
+        [ValidatePrototypeId<EntityPrototype>]
+        public const string AdminObserverPrototypeName = "AdminObserver";
+
         /// <summary>
         /// How many players have joined the round through normal methods.
         /// Useful for game rules to look at. Doesn't count observers, people in lobby, etc.