For BBRv2, - Change the default STARTUP and DRAIN cwnd gain to 2.0. - Change the meaning of BBQ2 to use 2.885 cwnd gain for STARTUP and DRAIN. Flag is default true since cwnd gain 2.0 has been tried in a finch experiment for a long time. Protected by FLAGS_quic_reloadable_flag_quic_bbr2_flip_bbq2. PiperOrigin-RevId: 336296878 Change-Id: I8e14586fe92313c0418d03ea5f4ffae610e2ece1