cmd/nginx-auth/nginx-auth: update auth to allow for new domains

With MagicDNS GA, we are giving every tailnet a tailnet-<hex>.ts.net name.
We will only parse out if legacy domains include beta.tailscale.net; otherwise,
set tailnet to the full domain format going forward.

Signed-off-by: nyghtowl <warrick@tailscale.com>
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