Increase UL MCS to 28 in 5G CU-DU F1 RFsim test

The 2nd next commit will introduce channel emulation in RFsim. Enable
higher MCS to be able to see retransmissions.

ci-scripts/conf_files/gnb-du.sa.band78.106prb.rfsim.conf

@@ -177,6 +177,8 @@ MACRLCs = (
     remote_n_portd  = 2153;
     pusch_TargetSNRx10          = 200;
     pucch_TargetSNRx10          = 200;
+    dl_max_mcs = 28;
+    ul_max_mcs = 28;
   }
 );