It isn't asking you to fix something.
It is warning you that your cluster is behaving in a particular way, and that if/when you upgrade to 7.0 it will behave in a different way.
This is about your cluster sending a response, and the format of that response is going to change in 7.0; It has nothing to do with the way you are configuring your hostnames or publish addresses, it's only about how ES formats them in particular REST responses.
You don't have to do anything, but if you have a client that relies on the current behaviour, then it will break when you upgrade to 7.0.
The ES server cannot tell whether you have such a client we don't know how those responses are being consumed, so the only option is to issue that warning on all clusters.
If you want to silence the warning, you can do so by making the change now (on 6.7) to format responses with the new (7.0) style of address. You do this by setting the es.http.cname_in_publish_address system problems as described above:
export ES_JAVA_OPTS="$ES_JAVA_OPTS -Des.http.cname_in_publish_address=true"
If you have clients that rely on the old style, then making this change could break them.