* protocols. The http/1.1 transport will never be dropped.
     *
     * If multiple protocols are specified, [ALPN][alpn] will be used to negotiate a transport.
     * Protocol negotiation is only attempted for HTTPS URLs.
     *
     * [Protocol.HTTP_1_0] is not supported in this set. Requests are initiated with `HTTP/1.1`. If
     * the server responds with `HTTP/1.0`, that will be exposed by [Response.protocol].
     *