Tag to be used with the main branch of atlas/athena. Update(s) since 2.1.21: - Updated CheckerGccPlugins: Improvements with/for GCC 14; - Fixed a configuration issue in the (standalone) XRootD build; - Similar to GeoModel, separated the headers of Acts, Gaudi, Geant4 and VecMem into their separate directories; - Updated Finctdaq-common.cmake to pick up more of tdaq-common's Python code; - Updated the external versions to the ones used in the Athena,main builds at the moment.