gfe-relnote: In QUIC, send a PING when skipping packet number PTO fires and there is no data to send. Protected by gfe2_reloadable_flag_quic_send_ping_when_pto_skips_packet_number. PiperOrigin-RevId: 300369878 Change-Id: I395da0107a24c218d9b96fc87df056a6213604f3