Test IPv6 dual stack

[rcelyte]

In [[current year]], IPv6 is still not ubiquitous across residential networks internationally, so clients cannot be assumed to support deeplinking to IPv6 endpoints. Additionally, accessing legacy master servers via domain names containing AAAA records will fail for IPv4-only clients, since the game's code doesn't know to fall back on failure (this shouldn't an issue for graph connections, with C♯'s more intelligent HTTPS client). BeatUpServer should detect IPv6 clients and respond with IPv6 deeplink endpoints in such cases, while continuing to provide IPv4 endpoints for those stuck in the stone ages. Care should be taken to ensure IPv4 clients are never misidentified and supporting IPv6. This will require extensive testing from those affected by the global stagnation of internet protocol adoption.