Fixes#7403.
Tested by enabling UPnP on the router, and checking on the router page
that the external ports of the UDP mappings match what is shown in the
logs and the internal ports matching the QUIC listening port.
The allowed IPv4 ranges are the same as before. But we now also accept IPv6 addresses in the ULA range FC00::/7. These addresses don't require an interface identifier and are roughly equivalent to the IPv4 private ranges.
Typical usecases:
VPN interface IPs: Wireguard, OpenVPN, Tailscale, ...
fixed IPv6 LAN addressing while the provider assigns a dynamic prefix. e.g used by pihole
https://cs.opensource.google/go/go/+/refs/tags/go1.21.0:src/net/ip.go;l=146