Support Geant4 10.3 to integrate with BACCARAT

[sethrj]

@MarkMurdy Let's track progress of the older Geant4 integration here.

In addition to managing changes to the Geant4 codebase, e.g.: https://github.com/celeritas-project/celeritas/blob/62ff58dffded582f1a71247d7840bdc912ae73b0/src/celeritas/ext/detail/OpticalPhysics.cc#L24

we may also need to extend FindGeant4.cmake to work with that old version.