Add readonly where it is missing and fix those field names according to their modifiers (#2589)

Content.Server/GameObjects/EntitySystems/AI/Steering/EntityTargetSteeringRequest.cs

@@ -9,7 +9,7 @@ namespace Content.Server.GameObjects.EntitySystems.AI.Steering
         public MapCoordinates TargetMap => _target.Transform.MapPosition;
         public EntityCoordinates TargetGrid => _target.Transform.Coordinates;
         public IEntity Target => _target;
-        private IEntity _target;
+        private readonly IEntity _target;
 
         /// <inheritdoc />
         public float ArrivalDistance { get; }