Update Geant4 version to 10.6.3.12

This is a follow up to !1115 (merged) and towards ATLASSIM-6058. Though !1115 (merged) enabled LTO builds of Geant4 itself without error, linking to these libraries in Athena's "big" libraries with LTO enabled there results in warnings related to Geant4 code, primarily maybe-uninitialized and alloc-size-larger-than. These have been fixed in upstream Geant4, with the patches applied to ATLAS' geant4 fork in tag https://gitlab.cern.ch/atlas-simulation-team/geant4/-/tags/v10.6.3.12. Checks with Athena and AthSimulation have shown these fixes do result in any changes to output hit data compared to v10.6.3.10/11 over samples of 100ttbar events.

The changes here simply point the Geant4 external to this new tag.