chore: switch geant4 to qt6

This PR switches geant4 visualization from Qt5 to Qt6, taking the installation of Qt from 270 MB down to 70 MB.

spack-environment/xl/spack.yaml

@@ -38,7 +38,7 @@ spack:
   - fmt
   - gaudi
   - gdb
-  - geant4
+  - geant4 +opengl
   - graphviz
   - hepmc3
   - heppdt