campaigns: update a migration with an UPDATE

Created by: LawnGnome

The boat burning migration that was merged in #13001 didn't account for the possibility that a campaign may have been created in the narrow time window between the campaign_spec_id and last_updated_at fields being added. Since the sum total of people who might be affected is likely three, and they all work here, we'll deal with this by updating the existing migration.

Neither @eseliger nor I think this needs to be cherry picked for 3.19.