Update default HTTP/3 priority. Default value in draft spec was changed from 1 to 3 at https://github.com/httpwg/http-extensions/pull/1172. Protected by FLAGS_quic_reloadable_flag_quic_http3_new_default_urgency_value. PiperOrigin-RevId: 329295352 Change-Id: I7a3a42ed25f3250b8aa465c5c34ca924ba3167bc