Depreciate uml (user-mode-linux) backend

[obbardc]

The uml (user-mode-linux) backend was really designed as an experiment to support machines without nested virtualisation. Since most cloud providers (and now GitHub actions!) are starting to support nested virtualisation in more machines, we can safely depreciate the uml backend.

In the next release, I'd like to:

• stop the uml backend from being automatically chosen
• print a warning when users manually choose the uml backend with a link to this ticket
• update documentation with the above information

In some future release (TBD) we will remove support for user-mode-linux entirely.