reduce sl_ahead to catch the RACH Msg1 in time in 24PRB test

a bit difficult to understand: sl_ahead=6 => we systematically miss the
rach detection in the scheduler because the scheduler runs more than 6
slots ahead the rx slot processing (here > 11 slots because 6 (DL and UL
slots) + 4 (RU_RX_SLOT_DEPTH, so UL only))

so, reducing sl_ahead works better than large sl_ahead because the idea
behind is: the scheduler of slot+sl_ahead will never create work to do
in a UL slot before slot+sl_ahead+X (as minimum k2 is X, in the 24PRB CI
test: 6)

ci-scripts/conf_files/gnb.sa.band78.24prb.rfsim.conf

@@ -196,6 +196,7 @@ RUs = (
   att_tx         = 0
   att_rx         = 0;
   bands          = [78];
+  sl_ahead=3;
   max_pdschReferenceSignalPower = -27;
   max_rxgain                    = 114;
   sf_extension                  = 0;