I added a similar comment already to the associated feature request. This feature seems or seemed to exist at least for Serv-U server since version 8.0.0.1 and we are using it since version 11 up to and including version 15. If it is currently not support, then we maybe work with a bug (but not a feature request) and should not touch the configuration, but it works excellent. Serv-U replies on port 80 with "302 Moved temporarily" containing the https fqdn.
Please see the release notes:
"Automatically redirect from HTTP to HTTPS when a secure connection is required and a corresponding HTTPS listener is found".
So we had set "require secure connection before login" in the server limits and settings section and enabled both the HTTP and HTTPS listeners within a domain on the same but only on one of the server IPv4 addresses only.
Just give it a try.