networkd: change UseMTU default to true. (#6837)

networkd: change UseMTU default to true. (#6837)

Typically when DHCP server sets MTU it is a lower one. And a lower than usual
MTU is then thus required on said network to have operational networking. This
makes networkd's dhcp client to work in more similar way to other dhcp-clients
(e.g. isc-dhcp). In particular, in a cloud setting, without this default
instances have resulted in timing out talking to cloud metadata source and
failing to provision.

This does not change this default for the Annonymize code path.