Externals Update, main branch (2023.10.04.)

Updated all projects to atlasexternals-2.0.171. The changes wrt. atlasexternals-2.0.170 are the following (atlasexternals@2.0.170...2.0.171):

• Made sure that Gaudi would build successfully after https://github.com/root-project/root/pull/11844, in LCG/dev3 (meant for ATLINFR-5071);
• Added FindTheP8I.cmake and Findcontur.cmake to AtlasLCG (meant for AGENE-2225, thanks to @ewelina and @pengwa).

The CI build should not be affected by any of those changes. 🤔 So as long as the normal incremental build succeeds, this should be safe to merge at any time.