Deprecation notice for dropping Unicorn support entirely in 14.0

As described in gitlab#300881 (comment 549722356), we are planning to drop Unicorn support in the 14.0 release.

As such, in 13.12 we'll need to display a deprecation warning during reconfigure if unicorn is in use as heads up that it'll be removed from 14.0. The warnings should become errors for the 14.0 release.

/cc @twk3