Tag to be used with the 21.X branches of Athena. Main updates in this tag: - Switched AthAnalysisExternals to LCG_93, and updated the standalone packages to build the same versions of the externals as what LCG_93 has; - Moved a bunch of source tar files to the new ATLAS EOS webserver; - Cleaned up the "find modules" a bunch, improving the suppression of locally installed libraries when using an LCG release; - The macOS builds now only build TGZ files with CPack by default; - Updated many packages to perform full/clean builds when they need to build a new version of their external;