Added foreign key validation for routes.namespace_id

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What does this MR do and why?

This MR validates the foreign key introduced in this MR synchronously.

References

• #525273 (closed)

How to set up and validate locally

1. Checkout to this branch
2. Run bin/rails db:migrate

MR acceptance checklist

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