MachineNic.Mac must not be marked as deprecated #202

Description

For machines we rely on the mac of a nic to configure the Interface renaming from for example eth0 -> lan0 and the mac is the only valid unique identifier to tell systemd-networkd which interface must be renamed.

With metal-stack/firewall-controller-manager#94 we rely on the MAC field to be able to decouple the firewall-controller from reading files on the machines to configure the network

This was done by mistake in #199