Tag to be used with the 21.X branches of atlas/athena. Updates in this tag: - Updated AtlasLCG to be compatible with LCG_94; - Upgraded all packages used by AnalysisBaseExternals to have the same version as they do in LCG_94; - Upgraded AthAnalysisExternals to build against LCG_94 by default; - Fixed a small issue in HDF5's build, where it wouldn't use ZLIB correctly from the LCG release; - Backported some technical updates from the master branch; - Added the ability to build Doxygen documentation using CMake once again.