gfe-relnote: Use PTO_RETRANSMISSION instead of PROBE_RETRANSMISSION for PTO retransmission. Also populate transport connection stats for debugging purpose. Protected by existing gfe2_reloadable_flag_quic_enable_pto. PiperOrigin-RevId: 277780869 Change-Id: I8e7584bebaf966ecf3fa7549767ef8bdf9d1f14d