We want to test our new IPv6 code in mixed networks, so let's add a `mixed+hs-v23-ipv6` network to chutney. It should be pretty much a copy-paste from `mixed+hs-v23` and `hs-v23-ipv6-md`. (There's no need for a non-md variant, all supported tor versions can use microdescriptors for IPv6 onion services.) Proposed design: * 2 dual-stack authorities, * 2 IPv4-only relays, * 2 dual-stack relays * 2 IPv6-only v2 onion services * 2 IPv6-only v3 onion services * 2 IPv6-only clients For each kind of node, one should be a new version, and one should be an old version. Other networks test IPv4-only authorities and clients, so we don't need them. (But we want to test IPv4 and dual-stack relays in the same network.)