Externals Update, 21.6 branch (2020.09.30.)

Updated all projects to atlasexternals-1.0.71. The full list of changes wrt. atlasexternals-1.0.66 is a bit lengthy (atlasexternals@1.0.66...1.0.71), but luckily most of it does not affect AthGeneration.

• Updated to using externals from LCG_97a in AnalysisBaseExternals and AthAnalysisExternals (does not affect 21.6);
• Updated FindCLHEP.cmake to set up a correct RPM dependency for LCG_97+ (does not affect 21.6);
• Added Findnlohmann_json.cmake to AtlasLCG for setting up correct RPM dependencies on that package (does not affect 21.6);
• Made FindUUID.cmake set up the correct LCG RPM dependency (it does affect 21.6);
• Taught yampl how to pick up uuid from the LCG release for its build (it does affect 21.6);
• Fixed the build of HDF5 with Xcode 12.0 (does not affect 21.6);
• Fixed the build of ROOT 6.20/06 on macOS against our privately built Python version (not not affect 21.6);
• Fixed the "platform detection" on macOS using modern CMake versions (does not affect 21.6);
• Eliminated a source of warnings while building AnalysisBaseExternals and AthAnalysisExternals with CMake 3.15+ (does not affect 21.6).
• Prevented AthenaExternals and AthDerivationExternals from picking up uuid from the LCG release, even if it would provide it (should not affect 21.6).

The main purpose of all of this is to fix the build issue(s) of AthGeneration for the x86_64-centos7-gcc62-opt platform. (https://bigpanda.cern.ch/nview/?nightly=21.6_AthGeneration_x86_64-centos7-gcc62-opt) As discussed in ATLINFR-3736. It would not have been necessary to use the very latest version of atlasexternals's 1.0 branch, but as long as I'm updating the externals for this branch, I might as well update to the latest and greatest...