Fix parent_links for moved epic_issues

We found a bug for syncing epic/issue relationships to work item parent links. It happens when an issue with an epic gets moved from one project to another one. In that case we didn't update the work item parent link association.

This bug only affects gitlab.com as we didn't rollout the syncing on self managed instances. However we need to fix this before releasing 17.1. For gitlab.com we also need to fix the affected records (theoretically this can happen after 17.1 is released).

Tasks

• Fix syncing the association. MR: !155113 (merged)
• Backfill to fix the wrong associations. Around 200 records are affected on production.
  • We need to create a parent work item link for every missing epic issue.
  • We need to delete all orphaned parent links