Use TOF v20a in the mCBM 2021 setup, until we get a double wall

setup/setup_mcbm_beam_2021_03.C

@@ -59,7 +59,7 @@ void setup_mcbm_beam_2021_03()
 //  TString muchGeoTag      = "v19a_mcbm";    // need 3 layers and RPC here
 //  TString trdGeoTag       = "v18q_mcbm";    // need to add TRD-HR module -> issue #1596
   TString trdGeoTag       = "v18q_mcbm";    // 2 TRD modules 
-  TString tofGeoTag       = "v21a_mcbm";    // double wall 
+  TString tofGeoTag       = "v20a_mcbm";    // double / triple stack
   TString richGeoTag      = "v20b_mcbm";    // geometry provided by RICH group
   TString psdGeoTag       = "v18d_mcbm";    // at 25 degree, below the beampipe
   // ------------------------------------------------------------------------