Don't encourage usage of `columns_hash`

As discussed in https://github.com/plataformatec/simple_form/pull/1094,
we should not encourage usage of `columns_hash`, and instead provide an
alternate method to determine whether or not an attribute exists.

The language `attribute` was chosen over `column` since these are in the
`AttributeMethods` module.
As a general principle, I think we should aim to require the least amount technical literacy required to comprehend these messages, because people need to understand them to take action (when I added [this](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/json/encoding.rb#L146-L155) message @jeremy made me add an example of "circular reference").

In this case though, my main concern is that "return a null object" sounds too similar to "returning nil", so it might be extra confusing in this context :)