Remove connection-specific headeres for HTTP/2 and HTTP/3

[MihaZupan]

Fixes #583

YARP workaround for dotnet/aspnetcore#24543 that was merged into 6.0 in Kestrel.

[Tratcher]

This handles h2 response protocol correctness but there are more scenarios to consider. https://tools.ietf.org/html/rfc7230#section-6.1 says we should remove the Connection header from all requests and responses, and all headers referenced by that (e.g. forwarding Connection: close would be bad for proxy scenarios). However, removing Connection: Upgrade would break WebSockets.

[Tratcher]

We also need to test what happens for requests. Does HttpClient filter out these headers on h2/3?

[MihaZupan]

This PR specifically fixes the Http2 client => Http11 backend scenario described in #583.

Should the connection header (#439) be dealt-with in the same PR?

[Tratcher]

This PR should also cover the request version of the #583 scenario.

Do some reading about the Connection header and decide if you want to handle that at the same time or as a separate issue/PR. I'd forgotten we had a separate issue to track that already.

[MihaZupan]

On the request side, HttpClient drops these headers:
https://github.com/dotnet/runtime/blob/45be6750943a043c15f2ce5e4700e48e63b5c7c0/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Connection.cs#L1143

[Tratcher]

That's good. HttpClient doesn't skip the Keep-Alive header?

[Tratcher]

The Keep-Alive question can be investigated separately.