New Geant4 externals: heptools-geant4ext20210906.cmake

Updates:

• ROOT 6.24/06
• VecCore 0.8.0
• VecGeom 1.1.17
• CLHEP 2.4.5.1
• pybind11 2.7.1

Note: I have placed tarballs for VecGeom, CLHEP, and pybind11 in lcgpackages/tarFiles/sources already.