Tag to be used with the 21.X branches of atlas/athena. Update(s) in this tag: - Made FindUUID.cmake set up the correct LCG RPM dependency; - Taught yampl how to pick up uuid from the LCG release for its build; - Fixed the build of HDF5 with Xcode 12.0; - Fixed the build of ROOT 6.20/06 on macOS against our privately built Python version; - Fixed the "platform detection" on macOS using modern CMake versions; - Eliminated a source of warnings while building AnalysisBaseExternals and AthAnalysisExternals with CMake 3.15+.