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.
Merge pull request #15976 from sgrif/sg-attribute-exists

Don't encourage usage of `columns_hash`