RHadrons:G4ProcessHelper - temporarily switch back to using CustomPDGParser

John Derek Chapmanrequested to merge
jchapman/athena:RHadronsCleanup_main into main

RHadrons:G4ProcessHelper - temporarily switch back to using CustomPDGParser as issues were found with the corresponding functions in TruthUtils - see original changes in !65434 (merged).

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