Reschedule migration to add admin_mode scope for PATs

What does this MR do and why?

The background migration was originally added in !107875 (merged), but it failed in certain installations. Token scopes are serialized by ActiveRecord and stored as strings, but our documentation contained a script which added a scope as symbol ➡ which led to the failing jobs.

This MR fixes the background migration to consolidate symbol values stored in scopes when needed, and requeues the migration.

Related to #391598 (closed)

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