Deprecate Instance.DataCost to hide it from documentation

As a developer, it is inconvenient to see this member pop up in every class reference on the Developer Hub, since this property is not used by any subsystem of the engine that isn’t deprecated (as far as I know).

The property should be marked as deprecated so that it doesn’t show up in the documentation anymore.

If Roblox were able to address this request, it would cut down space needed for displaying the API references by a little bit and reduce potential of confusing less experienced developers with the existence of this property.