Follow-up from "Add Sample Data" - Move override_date_attributes to RelationFactory

The following discussion from !41699 (merged) should be addressed:

• @ayufan started a discussion: (+4 comments)

  If I understand that correctly this is meant to change data_hash object that later gets used. This likely requires a comment why it is done like that.

  However, a general pattern established is to rather use RelationFactory, maybe doing something like: Gitlab::ImportExport::Project::Sample::RelationFactory and implementing a method def setup_models that would overwrite @relation_hash.