Distinguish the way we adjust the stepper for compatibility with Geant4 version greater than 10.4

This is a temporary solution to allow Athena to compile and run successfully with Geant4.10.6. After discussions with John Apostolakis, and @mmuskinj we agreed that for the time being we can create and adjust the stepper in this way (waiting to have the possibility to use a cleaner way of setting the stepper - implemented in !30394 (merged) - because this needs some debug on the Geant4 side). This change is needed because of interface changes in 10.6.

We keep for now the old 'getStepper' method, even if the two solutions should compile with any Geant4 version - and not change the output. This is useful to see if the output doesn't change and we can use safely the new implementation for now.