Replace index on Attestation project id, internal id with unique index

What does this MR do and why?

Creates a post-deployment migration to drop index_slsa_attestations_on_project_id_iid, and replace it with the same index but unique.

This index possibly should have been made unique in !206787 (merged). The slsa_attestations table is not yet used in production, and is expected to have few rows if any.

Related to #573353