gfe-relnote: fix path probe request in IETF QUIC, padded ping with peer address change should not be treated as path probe request. Only a packet contains PATH_CHALLENGE frame is considered as a path probe request. Protected by disabled QUIC version 99. This CL also changes UpdatePacketContent to no-op in IETF version. PiperOrigin-RevId: 296970477 Change-Id: Ida8c7e6a686c08508b936081c812cd14869c2137