[OVNK] ARP doesn't exist for v6: https://github.com/j-keck/arping/

https://github.com/j-keck/arping/blob/c50f4319d307d95d1e2b344c4504158705e919de/arping.go#L227 doesn't support IPV6

We use it here:

func GetMACAddressFromARP(neighIP net.IP) (net.HardwareAddr, error) { 
   hwAddr, _, err := arping.Ping(neighIP)   
   if err != nil { 
       return nil, err   
   }    
   return hwAddr, nil
}

and so we get errors like:
   2022-08-09T20:35:57.615015621Z E0809 20:35:57.614974   33417 node.go:664] Failed to lookup hardware address for gatewayIP fd13:f100:e876:a09::4: not a valid v4 Address: fd13:f100:e876:a09::4

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Silence those logs for v6 and add a TODO: Implement IPV6