SPI-2512: LCG_104d_ATLAS_5_6

All changes wrt LCG_104d_ATLAS_3_4:

• evtgen-2.2.1 -> evtgen-2.1.1
• herwig++-7.3.0 -> herwig++-7.2.3p2 (thepeg-2.3.0->thepeg-2.2.3)
• madgraph5amc-3.5.3.atlas4 -> madgraph5amc-3.5.1.atlas3
• pythia8-310 -> pythia8-308

SPI-2512