udev README.Debian: Update transitional rules and mention *.link files

 - 01-mac-for-usb.link got replaced with 73-usb-net-by-mac.rules

 - /etc/systemd/network/50-virtio-kernel-names.link is an upgrade
   transition for VMs with virtio

 - Describe *.link files as a simpler/less error prone (but also less
   flexible) way of customizing interface names.

Closes: #868002