Tag to be used with the master/22.0 branch of atlas/athena.
Update(s) since 2.0.135:
  - Made FindGeant4.cmake explicitly set up the (private) XercesC dependency of the G4 (static) libraries;
  - Introduced the FindActs.cmake module, to ensure that the runtime environment of Acts would be set up correctly when it's not taken from AthenaExternals;
  - Made mergeConfdb2.py fail on syntax errors instead of (fairly) silently swallowing them;
  - Updated AthDerivationExternals to include Acts and GeoModel;
  - Taught AtlasCMake and AtlasLCG how RHEL identifies itself in its /etc/os-release file.