xe-guest-utilities - update documentation.

It seems that xe-guest-utilities is no longer maintained as a standard package for Ubuntu related distros. Last release it was available for is Focal Fossa. See https://launchpad.net/ubuntu/+source/xe-guest-utilities

See also advice from OliverLambert on this forum here:- https://xcp-ng.org/forum/topic/5870/xe-guest-utilities-on-ubuntu-22-04?_=1661366159852

If that situation is going to be permanent, then the relevant documentation installing xe-guest-utilities on Linux/Ubuntu distros should be updated to remove existing advice to install via sudo apt install
See documentation at:-
https://xcp-ng.org/docs/guests.html#linux

Thanks