Use project_id as sharding key for packages_dependencies table

What does this MR do and why?

Use project_id as sharding key for packages_dependencies table

• Finalize the background migration to backfill project_id in the packages_dependencies table.
• Add project_id validation present to the Packages::Dependency model.
• Remove temporary code to search and update existing packages dependencies without project_id.
• Add NOT NULL constraint to the project_id column in the packages_dependencies table.
• Use project_id as a sharding key for the packages_dependencies table.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No.

How to set up and validate locally

I don't think we have something to validate here beside running the migrations.

Related to #465276 (closed)