Update docs to more recent OS versions

[marado]

The docs seem to imply that xenial is the latest Ubuntu supported version, but http://packages.arctica-project.org has repos for focal.

Equally, there are repositories for Debian bullseye an bookworm, but the documentation speaks of Debian 9/stretch.

[FlachyJoe]

Hi,
ArcticaGreeter_RemoteLogon-HowTo.md seems old too:

We are working on native Arctica support and plan to support FreeRDP logins to remote Windows Servers, as well.

but https://github.com/ArcticaProject/lightdm-remote-session-freerdp2