Consider enabling IPv4 / IPv6 dual-stack by default

[one-three-three-seven]

Currently, Grandine does not listen for IPv6 connections with the default settings.
Enabling the IPv6 listener is already very easy with the --listen-address-ipv6 :: argument.
Please consider enabling the IPv6 listener by default to improve the network's overall IPv6 readiness.

I suggest informing users about this change in the release notes if you decide to implement it. This change will open ports 9050 (UDP6/TCP6) and 9051 (QUIC) by default, which may already be in use on the user's system or blocked by a firewall.

JFYI: sigp/lighthouse#2023 (comment)

[totoCZ]

With

--listen-address-ipv6 :: --libp2p-port-ipv6 9000 --discovery-port-ipv6 9000 --quic-port-ipv6 9001

I am able to use just two ports. 9000/tcp 9000/udp 9001/udp on both v4/v6.