Externals and CMake Configuration Update, main branch (2023.10.08.)

Updated all projects to atlasexternals-2.0.172. The changes wrt. atlasexternals-2.0.171 are (atlasexternals@2.0.171...2.0.172):

• Multiple tweaks to FindGaudi.cmake:
  • Set helper variables for Gaudi/ROOT to find VDT;
  • Avoid CMake errors if Gaudi is not found;
  • Handle find_package keyword arguments correctly;
• Replaced the error messages in the converter generating code with warnings in AtlasLibraryFunctions.cmake, when Gaudi is not found;
• Made atlas_add_test(...) more resilient against using -NOTFOUND variables in its arguments.

The VDT update is necessary for the dev3LCG nightly, which continues to be in trouble, as discussed in ATLINFR-5071. All the others are there to make the Athena build possible even when any one of (or all of) Gaudi, tdaq and/or tdaq-common are missing. Just to make our experimental nightlies a little more robust going forward.

For the latter purpose I also removed the REQUIRED flag from the projects when looking for Gaudi, tdaq and tdaq-common. And also synchronized how they would all look for their base projects. (This was done a bit differently for the different projects so far.)

Finally, removed a hack from Athena/CMakeLists.txt that was put in for ATLASRECTS-6680, and should have been removed a long time ago. (ROOT doesn't depend on CUDA in the ATLAS LCG layers since a very long time.)

All updates are "incremental build friendly", the full-build label is just to be extra careful. Since some core CMake code is touched by this MR.