increase prach_dtx_threshold to 20 dB to avoid fake PRACH detections

ci-scripts/conf_files/gnb.band78.sa.fr1.106PRB.usrpn310.conf

@@ -237,7 +237,7 @@ L1s = (
 {
   num_cc = 1;
   tr_n_preference = "local_mac";
-  prach_dtx_threshold = 120;
+  prach_dtx_threshold = 200;
 #  pucch0_dtx_threshold = 150;
 }
 );