Update CI to LCG_99 / Geant4 10.7
This updates the CI to use LCG_99 with Geant4 10.7. We here also drop support for SLC6 and add a first preview for Centos 8 instead.
This is still a draft because the macOS runners couldn't cope with AppleClang 12.0 yet. @mpetric is updating/testing.
This fixes #201 (closed)
Edited by Simon Spannagel