Listen on IPv6 by default

[tomaka]

cc #5687
We had to revert listening by default because of benign libp2p errors.

[tomaka]

To expand a little bit, there are two issues:

• My logs are spammed with warnings when listening on IPv6: On a Google Cloud node, listening on an IPv6 address triggers a lot of listener errors libp2p/rust-libp2p#1552

• Before Revert listening on IPv6 by default #5687, we were trying to listen both on /ip4/0.0.0.0/tcp/<port> and /ip6/::/tcp/<port>. This would try to open two sockets, and one of them would fail to open because of the port conflict.
  It should be enough, normally, to only listen on IPv6, right? The kernel would then also automatically accept IPv4 connections on that port, if I'm not mistaken.

[bkchr]

We probably need to set this this to false.

[twittner]

We probably need to set this this to false.

By default IPV6_V6ONLY is false on Linux and true on Windows (according to https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-ipv6-socket-options). It is probably a good idea to explicitly set this option to false to ensure consistent behaviour accross platforms.

[twittner]

I have opened libp2p/rust-libp2p#1555. Instead of setting IPV6_V6ONLY to false I set it to true. It allows us to bind two sockets to the same port, avoids mixed comparisons and works on more operating systems (on those that do not (want to) support IPv4-mapped IPv6 addresses such as OpenBSD).