Unfortunately we can't enforce the version in the gemspec because
responders only supports Rails 5.2 now, and Devise still supports
previous versions.

We'll drop support for those in a future major release, so for now I'm
not adding any version.

This also adds a warning in case someone is using an older version of
responders and tries to set the error/redirect statuses via Devise, so
that they know what to do (upgrade responders) in that case.