Externals Update, 21.2 branch (2020.09.24.)

Updated all projects to atlasexternals-1.0.70. The list of changes wrt. atlasexternals-1.0.69 are the following (atlasexternals@1.0.69...1.0.70):

• Made FindUUID.cmake set up the correct LCG RPM dependency (not really relevant for 21.2);
• Taught yampl how to pick up uuid from the LCG release for its build (not relevant for 21.2);
• 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;
• Eliminated a source of warnings while building AnalysisBaseExternals and AthAnalysisExternals with CMake 3.15+.

As you can see, most of the updates are here to make sure that AnalysisBase would be possible to build on the latest version of macOS.

@kpachal, @krumnack, if we could still get this into 21.2.141, that would be great. But if it doesn't finish in time, we could also live without it...