release/21.6.43

Ewelina Maria Lobodzinska

@ewelina

65cda140 · Merge branch '21.6' into '21.6' · Sep 11, 2020

contains MR:
update to Pythia8.303 https://gitlab.cern.ch/atlas/athena/-/merge_requests/36276
Fixining conflicts in UserPhotonFlux from Pythia8.303 https://gitlab.cern.ch/atlas/athena/-/merge_requests/36367