Fix HepMcParticleLink construction in HGTD_Digitization
Update of the HGTD_Digitization, following !35585 (merged) Should fix issue with broken truth-link in pile-up samples.
The implementation in master already uses the proper HepMcParticleLink construction, as well as the HGTD_FastDigitization, both in 21.9 and master.
FYI @aleopold