ipaserver: Update README with detailed Ubuntu support

Ubuntu does not have a FreeIPA server package since version 20.04. As
versions 16.04 (Xenial Xerus) and 18.04 (Bionic Beaver) will be
supported by Canonical until 2026 and 2028, repectively, we should keep
existing support for both versions in the ipaserver, ipareplica and
ipabackup roles until them.

This patch changes documentation to reflect that only those versions are
supported.