Fix HepMcParticleLink construction in HGTD_Digitization

Thomas Streblerrequested to merge
tstreble/athena:HepMcPartLink_HGTD into 21.9

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

Merge request reports