Move to const DataHandle

This MR moves to using const DataHandles to retrieve the CscIdHelper (following a fix in 21.3: !28283 (merged)).

This MR should go in ideally before building the next 21.0 release, thus adding the urgent label.

Adding @jchapman and @pottgen

MuonSpectrometer/MuonCnv/MuonEventAthenaPool/src/CscRawDataCnv_p4.cxx

@@ -6,7 +6,6 @@
 #include "CscRawDataCnv_p4.h"
 #include "GaudiKernel/GaudiException.h"
 #include "StoreGate/StoreGateSvc.h"
-#include "MuonIdHelpers/CscIdHelper.h"
 #include <TString.h> // for Form
 
 bool CscRawDataCnv_p4::initialize() {