Migrate MCTruthClassifier to use TruthUtils PDG ID helper functions

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

Minimal changes, just replacing comparisons to hard-coded numbers with comparisons to constants or use of helper functions from TruthUtils/AtlasPID.h.

Further clean-up will be done in a separate MR.

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