Merge branch '21.6-mg' into '21.6'

update to Pythia8.303 and go back to MG2.7.3

See merge request !36276

Projects/AthGeneration/externals/MadGraph5Amc.cmake

@@ -2,6 +2,6 @@
 # File specifying the location of MadGraph to use.
 #
 
-set( MADGRAPH5AMC_LCGVERSION 2.8.0p3.atlas6)
+set( MADGRAPH5AMC_LCGVERSION 2.7.3p3.atlas6)
 set( MADGRAPH5AMC_LCGROOT
    ${LCG_RELEASE_DIR}/MCGenerators/madgraph5amc/${MADGRAPH5AMC_LCGVERSION}/${LCG_PLATFORM} )