Finalize fork_networks organization_id backfill

What does this MR do and why?

This MR Finalizes fork_networks organization_id backfill migration !184051 (merged) that is part of the list of tasks to add a sharding key to fork_networks

The current query in postgress AI shows no new records are being created with a NULL organization_id on fork networks: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/38492/commands/118364

The chatops progress is also completed

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Related to #535998 (closed)