Finalize backfill of security_scans.project_id and set NOT NULL for sharding_key

Related to #454948 (closed) we still need to finalize the migration and set NOT NULL before we can mark this as the sharding_key. Also see !171076 (comment 2186393876) .

As discussed at &13087 (comment 2186465650) this work should be handled by grouptenant scale likely using Housekeeper. This may require some tweaks to Housekeeper as this does not have a desired_sharding_key set.