Add Background Migration to Backfill Root Storage Statistics Fork Sizes

What does this MR do and why?

Add background migration to backfill root storage statistics fork sizes

Backfill the following columns for namespace_root_storage_statistics:

• public_forks_storage_size
• internal_forks_storage_size
• private_forks_storage_size

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/399372

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