Externals Update, master branch (2023.06.29.)

Updated all projects to atlasexternals-2.0.158. The changes wrt. atlasexternals-2.0.156 are (atlasexternals@2.0.156...2.0.158):

• Updated FindLhapdf.cmake to set up the use of the ATLAS specific LHAPATH under all circumstances (pinging @ewelina);
  • This was necessary to make sure that paths in $LHAPATH would be in the correct order in all of our projects. And now that FindLhapdf.cmake does this itself, I simplified the Superchic_i package a bit, which was previously extending $LHAPATH itself. (In as it turns out a fragile way.)
• Modernised the builds of VecGeom and Geant4, and updated how they would be found by the offline code (thanks to @bmorgan);
  • This will be important once we want to make use of Celeritas (or AdePT 😉);
• Introduced C++20 fixes for: Geant4, GeoModel and Coin3D for ATLINFR-4959 (thanks to @elmsheus).

The changes are not completely trivial, so full-build, full-unit-tests and full-integration-tests are all in effect! And the RC should plan on a CI node cleanup after merging this one in. 😉