Externals Update, main branch (2023.09.13.)

Updated all projects to atlasexternals-2.0.169. The changes wrt. atlasexternals-2.0.168 that are relevant for this repository (atlasexternals@2.0.168...2.0.169), are:

• Updated GDB to version 13.2, while accounting for dependency changes in resolveAtlasAddr2Line;
  • Needed to fix an issue on aarch64-el9-gcc11-opt, thanks to @elmsheus;
• Removed many outdated AtlasLCG modules (thanks to @fwinkl);
• Made Findsetuptools.cmake more robust for finding the module in different LCG versions (thanks to @fwinkl);
• Made AtlasCMake and AtlasLCG use C++20 with GCC 13+ by default;
• Improved/fixed the C++20 build support in Boost, Eigen and Geant4;
• Made CheckerGccPlugins ignore (additional) standard violations in code generated by GoogleTest, and switched it to using egrep instead of grep (thanks to @ssnyder).

None of these changes should affect our CI builds, but let's do a full suite of tests just to be sure. Just because the build code was tweaked in some tricky ways in some places.