Migration to release 21

Pointers with more info on migration

• MG5 releases: https://twiki.cern.ch/twiki/bin/viewauth/AtlasProtected/MadGraph5aMCatNLOForAtlas#Release_history
• Blacklisted releases: https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/blob/master/common/BlackList_caches.txt
• Prepare and test MC: https://twiki.cern.ch/twiki/bin/viewauth/AtlasProtected/MCTopRequestHowTo
• Prepare and test MC part 2: https://twiki.cern.ch/twiki/bin/viewauth/AtlasProtected/PmgMcSoftware
• MC transform: https://twiki.cern.ch/twiki/bin/viewauth/AtlasProtected/PmgMcSoftware#Production_transforms_and_job_op
• Examples: https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/tree/master

H+ Jira ticket https://its.cern.ch/jira/browse/ATLMCPROD-8349