In order to fix the ART tests, some changes already available in 21.6, need to be added to master. In this MR some packages needed for Pythia8.3xx are installed in External/Pythia8/CMakeLists.txt and MultiBjetFilter is added to GeneratorFilters.
Additionally, GenerateTruthJets.py file is corrected to take into account the changes introduced in JetRec package in master (and not present in 21.6)