do not embed http.Server in http3.Server (#3397)

This change removes the embedded http.Server struct from http3.Server.
It slightly changes some error behavior, in particular, it mandates
TLSConfig for methods that create QUIC listeners.

Before this change, only Addr, TLSConfig, Handler and MaxHeaderBytes
options were used from the http.Server. These are now defined directly
in http3.Server with an improved documentation.

example/main.go

@@ -185,7 +185,8 @@ func main() {
 				err = http3.ListenAndServe(bCap, certFile, keyFile, handler)
 			} else {
 				server := http3.Server{
-					Server:     &http.Server{Handler: handler, Addr: bCap},
+					Handler:    handler,
+					Addr:       bCap,
 					QuicConfig: quicConf,
 				}
 				err = server.ListenAndServeTLS(testdata.GetCertificatePaths())