samples/bluetooth/central_multilink: Relax RSSI requirement

The sample was only willing to connect if the pair RSSI level
was over -35dBm. That is ok when testing physically keeping
devices at less than 1 meter from each other.
But in CI we test with a default attenuation of 60dB between
devices and the Tx power is 0dBm.
Which causes the tests based on this sample to fail.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>