Interoping draft 20 HTTP/3

In the IETF 104 session today we discussed the next interop target. I agree
that aiming for draft 20 is a good target and anecdotally there is strong
interest from HTTP/3 implementations to do so.

Some comments were made that the changes to 20 were minimal. However, IMO
the changes to HTTP/3 are pretty big - the change to variable length
integers affects most types. It was discussed at length (relatively
speaking for HTTP/3), agreed and landed soon after. Unfortunately there are
some issues that gained consensus in Tokyo but have yet to be addressed;
either no PR or unmerged PR.

Therefore, I encourage us to close out some of the remaining issues on
HTTP/3 and QPACK before cutting new releases of those documents.

Lucas