gfe-relnote: (n/a) When QUIC switches from BbrSender to Bbr2Sender, let Bbr2Sender copy the bandwidth sampler states from BbrSender. Protected by --gfe2_reloadable_flag_quic_bbr_copy_sampler_state_from_v1_to_v2. PiperOrigin-RevId: 303846658 Change-Id: I37d88015801392608d87140213d01db02d01209f