Use direct G4Tubs query method

Building AthSimulation against Geant4 with VecGeom solids highlighted
use of a deprecated call to the GetDz method of G4Tubs.

Replace use of GetDz with recommended GetZHalfLength. No change in
behaviour as GetDz returns the value returned by GetZHalfLength. No
Geant4 version checks are used, as this behaviour has remained the same
since Geant4 1.0!