networkd: vxlan require Remote= to be a non multicast address (#8117)

Remote= must be a non multicast address. ip-link(8) says:
> remote IPADDR - specifies the unicast destination IP address to
>                      use in outgoing packets when the destination link layer address
>                      is not known in the VXLAN device forwarding database.

Closes #8088.

(cherry picked from commit bf443be9)
(cherry picked from commit 064feb82)