Prepare for new GaudiPartProp in Gaudi v38

gaudi/Gaudi!1493 (merged) replaces the old GaudiPartProp with a backport of LHCb Kernel/PartProp. This deprecates headers like GaudiKernel/ParticleProperty.h (which is not really needed in LHCb) and introduces the namespace Gaudi::Decays that might clash with Decays in some cases.

This backward compatible MR is to make sure that when we switch to Gaudi v38 everything still works.