Use project_id as sharding key for packages_nuget_symbols

What does this MR do and why?

Use project_id as sharding key for packages_nuget_symbols

• Finalize the background migration to backfill project_id in the packages_nuget_symbols table.
• Add project_id validation present to the Packages::Nuget::Symbol model.
• Add NOT NULL constraint to the project_id column in the packages_nuget_symbols table.
• Use project_id as a sharding key for the packages_nuget_symbols table.

References

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• Set sharding key for packages_nuget_symbols table (#461803 - closed)

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Related to #461803 (closed)