A quick workaround for that is to use one of the DNS servers from https://nat64.net/. There are also people running reverse proxies specifically for GitHub, e.g. https://danwin1210.de/github-ipv6-proxy.php.

(Ideally your ISP would be running NAT64 for you, especially if it's a VPS provider only giving you v6, but for whatever reason few of them do...)