Post UI Freeze change

[Brian Murray]

As documented in bug 1756862[1] the option to upgrade from an old release
of Ubuntu to Ubuntu 18.04 when booting the desktop Live CD installer has
been removed. This was done because the process is currently crashing
and if it fails it leaves the user in a very bad state.

I had a brief look for any documentation regarding the ubiquity upgrade
option and did not see any, but wanted to let you know in case I missed
something.

[1] http://launchpad.net/bugs/1756862

Thanks!
--
Brian Murray