gfe-relnote: In gQUIC, move dynamic stream accounting to LegacyStreamIdManager. Protected by gfe2_reloadable_flag_quic_stream_id_manager_handles_accounting. Making it clear that QuicSession::GetNumOpenIncomingStreams and QuicSession::GetNumOpenOutgoingStreams are only used for gQUIC. PiperOrigin-RevId: 308126943 Change-Id: Ib57ab52bf023ea83e1ada418ced8cb1dd08d186a