downgrade lmvm much

changes the default lmvm much from v23a to v20c as per discussion in https://redmine.cbm.gsi.de/issues/3135

setup/setup_sis100_muon_lmvm.C

@@ -41,7 +41,7 @@ void setup_sis100_muon_lmvm()
   TString magnetGeoTag    = "v22b";
   TString pipeGeoTag      = "v21d:v21i";
   TString stsGeoTag       = "v22d";
-  TString muchGeoTag      = "v23a_sis100_1m_lmvm";
+  TString muchGeoTag      = "v21c_sis100_1m_lmvm";
   TString trdGeoTag       = "v20c_1m";
   TString tofGeoTag       = "v21a_1m";
   TString platGeoTag      = "v22b";