iperf2 2.0.13, used in Ubuntu 20, does not need a "back channel"; we can
therefore use it in feMBMS (which does a broadcast, without back
channel). Ubuntu 22 comes with 2.1.5, which seems to require a back
channel, or at least, we cannot use it reliably in feMBMS. Hence,
downgrade to older iperf2 version.

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>