Fix the reading of HepMcParticleLink_p1 in 21.3

Even in 21.0 HepMcParticleLinks pointing to GenEvents which aren't the first entry in the McEventCollection are using the event numbers rather than the position McEventCollection as the eventIndex. This is a manual sweep of !31000 (merged) from master.

FYI @tlari, @jassahsa, @vpascuzz, @golling

added sweep:from master label

This merge request affects 1 package:

• Generators/GeneratorObjectsTPCnv

added 21.3 Generators review-pending-level-1 labels

CI Result SUCCESS (hash 39918099)

	Athena
externals
cmake
make
required tests
optional tests

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For experts only: Jenkins output [CI-MERGE-REQUEST 42995]

Build warning unrelated to this change and has already been fixed in !31104 (merged)

added review-approved label and removed review-pending-level-1 label

mentioned in commit d79f7101

merged

added sweep:ignore label

mentioned in merge request !31887 (merged)

mentioned in commit a2a70552

mentioned in merge request !35051 (merged)

mentioned in commit d7d950e8

mentioned in commit f5a0ad1f

mentioned in merge request !35087 (merged)

mentioned in commit 9cb917b4

mentioned in merge request !35241 (merged)

mentioned in commit ff887f72

mentioned in commit 5f6f8fcb