Tag to be used with the main branch of atlas/athena. Update(s) since 2.1.12: - Upgraded GeoModel to version 5.3.0 (following the update already done in atlas/athena); - Switched the CI to use LCG_104d_ATLAS_9; - Set CMP0069 to NEW, to allow for convenient link time optimizations in the ATLAS projects; - Upgraded Geant4 to version 10.6.3.11; - Turned on LTO for the Geant4 and VecGeom builds.