Geo: Add unique constraint to registry foreign keys

There are a number of registry tables which do not have a uniqueness constraint on their foreign keys:

• container_repository_registry
• job_artifact_registry
• merge_request_diff_registry
• package_file_registry
• terraform_state_version_registry

Adding the uniqueness constraints would reduce the risk of bad data.

Proposal

• Add post migrations deleting duplicate rows in the affected tables
• Add post migrations adding the new indexes with the uniqueness constraint (with a different name?)
• Add post migrations removing the existing non-unique foreign key indexes
• Add unique: true where needed in the doc https://docs.gitlab.com/ee/development/geo/framework.html => Opened !51185 (merged)