http3: fix listening on both QUIC and TCP

[mojatter]

Hi, I found an issue with the -tcp option in example not working.

% cd example
% go run ./main.go -tcp

% cd example/client
% go run ./main.go -q https://localhost:6121/demo/tile
GET https://localhost:6121/demo/tile
client Starting new connection to localhost ([::]:56076 -> 127.0.0.1:6121), source connection ID (empty), destination connection ID 3b2948435331ca22, version v1
Got response for https://localhost:6121/demo/tile: &http.Response{Status:"404 Not Found", StatusCode:404, Proto:"HTTP/3.0", ProtoMajor:3, ProtoMinor:0, Header:http.Header{"Content-Type":[]string{"text/plain; charset=utf-8"}, "X-Content-Type-Options":[]string{"nosniff"}}, Body:(*http3.hijackableBody)(0x14000298040), ContentLength:-1, TransferEncoding:[]string(nil), Close:false, Uncompressed:false, Trailer:http.Header(nil), Request:(*http.Request)(nil), TLS:(*tls.ConnectionState)(0x140002aa000)}
Response Body: 19 bytes
client Closing connection with error: Application error 0x100
client Connection 3b2948435331ca22 closed.

As shown above, a request for QUIC returns 404 Not Found.
The cause of the problem is that the http3.Server is not configured with a Handler.

Applying this pull request will return 200 OK as follows.

% go run ./main.go -q https://localhost:6121/demo/tile
GET https://localhost:6121/demo/tile
client Starting new connection to localhost ([::]:64885 -> 127.0.0.1:6121), source connection ID (empty), destination connection ID e35d6be0ca052bac51b466c0717d63a9df45f5b9, version v1
Got response for https://localhost:6121/demo/tile: &http.Response{Status:"200 OK", StatusCode:200, Proto:"HTTP/3.0", ProtoMajor:3, ProtoMinor:0, Header:http.Header{}, Body:(*http3.hijackableBody)(0x14000032080), ContentLength:-1, TransferEncoding:[]string(nil), Close:false, Uncompressed:false, Trailer:http.Header(nil), Request:(*http.Request)(nil), TLS:(*tls.ConnectionState)(0x14000102000)}
Response Body: 83 bytes
client Closing connection with error: Application error 0x100
client Connection e35d6be0ca052bac51b466c0717d63a9df45f5b9 closed.

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[marten-seemann]

Sorry for the long delay! This looks good to me.

[marten-seemann]

@mojatter Can you please sign the CLA (see #3465 (comment)). We'd need that so we can merge the PR.

[mojatter]

@marten-seemann Thank you. I fixed it.

[marten-seemann]

Thank you!