There shouldn't be anything preventing the TRRServiceChannel from using HTTP/3, but current telemetry shows it's never being used.

Although there's a HTTPS record for mozilla.cloudflare-dns.com, we never end up using that, as we need DoH to resolve the HTTPS record.
The alternative is the Alt-Svc header, but it seems the cloudflare DoH servers don't send that back in the response.

I did try using https://dns.google/dns-query which does return an Alt-Svc header, but for some reason the DoH connection isn't using it.

Ultimately, we have two issues here:

• Make sure the TRRServiceChannel can use the alt-svc. We can probably add unit test for that.
• Make sure we can resolve HTTPS records using native DNS. This seems like a rather extensive project.