Draft: Backfill project_authorizations_for_migrations table

What does this MR do and why?

Backfills the project_authorizations_for_migrations table introduced by !190374 (merged).

We need to restore a deleted unique index on project_authorizations over (project_id, user_id) (#418205). To do so, first we need to deduplicate the table since some installations may contain multiple rows per (project_id, user_id) combination with differing access levels. To deduplicate the table, we create the temporary project_authorizations_for_migrations which we will later swap-rename to project_authorizations.

In !190374 (merged) we already added triggers on project_authorizations to forward writes to project_authorizations_for_migrations.

References

• #418205
• #525999

How to set up and validate locally

1. Queue the background migration:

   bin/rails db:migrate:redo:main RAILS_ENV=development VERSION=20250507144731

2. Write a duplicate (user_id, project_id) row to project_authorizations with two differing access levels

3. Finalize the background migration:

   bin/rake gitlab:background_migrations:finalize[MigrateProjectAuthorizations,project_authorizations,id,'[]']

4. Verify the row in project_authorizations_for_migration has the lower access level

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Related to #525999