This base platform has recently been exchanged with Ubuntu as the underlying distribution. Since yesterday there is now a Kolab-Server-2.2.4 available as an image to be started and used for quick testing. You can expect all upcoming server version to be also based on Ubuntu. This does of course include the Kolab-Server-2.3 that will hopefully be released soon.
Instructions on how to use the Kolab Server images can be found in the Kolab wiki.