Tag to be used with the master branch of atlas/athena.
Update(s) since 2.0.106:
  - Updated AnalysisBaseExternals to use external versions in sync with LCG_100;
    * Introduced simple patches for KLFitter and BAT to make them compatible with ROOT 6.24/00;
    * Upgraded Boost to version 1.75.0;
    * Upgraded HDF5 to version 1.10.6;
    * Upgraded libxml2 to version 2.9.10;
    * Upgraded nlohmann_json to version 3.9.1;
    * Upgraded Cython to version 0.29.21 and pip to version 21.0.1 (both as part of the PyAnalysis package);
    * Upgraded XRootD to version 5.1.1;
      * Building OpenSSL 1.1.1 for XRootD, if it is not available on the build system already;
    * Upgraded ROOT to version 6.24/00.
  - Introduced the PRIVATE_WORKING_DIRECTORY option for atlas_add_test(...);
  - Made FindGaudi.cmake compatible with the latest Gaudi master version.