HTTP/3: Remove non-final h3 ALPNs

[JamesNK]

Should probably only have h3 when .NET 7 RTM ships.

[Tratcher]

https://github.com/dotnet/aspnetcore/pull/34968/files#r682486016

It's only kept right now because of curl.

[wtgodbe]

So this is just about removing h3-29 here once curl http3 tool recognizes h3? Do we know when that'll happen?

[Tratcher]

We should remove it before RTM regardless, but we'll wait until the last minute while we're testing with tools like curl.

[adityamandaleeka]

Still waiting for clients, but we should do this before end of 7.