Remove foreign key constraints to namespace_id & project_id from p_duo_workflow_checkpoints

What does this MR do and why?

Remove foreign key constraints to namespace_id & project_id from p_duo_workflow_checkpoints

• Because dropping the old partitions creates incidents due to projects and namespaces being high traffic tables (see issues)
• Because we're dropping older data, we don't really need the data integrity provided by FK constraints

References

• Related to Dropping partitions on p_duo_workflows_checkpoi... (#572109 - closed)
• Related to Avoid recurring lock contention due to add/drop... (#560503 - closed)
• https://docs.gitlab.com/development/database/foreign_keys/
• https://docs.gitlab.com/development/migration_style_guide/

Database migrations

See gitlabdotcom testing output

How to set up and validate locally

Run the migrations and in psql use \d p_duo_workflow_checkpoints to see FK have been dropped

bin/rails db:migrate:up:main VERSION=20251006061109
bin/rails db:migrate:up:main VERSION=20251006061128

Run the migrations and in psql use \d p_duo_workflow_checkpoints to see FK have been restored

bin/rails db:migrate:down:main VERSION=20251006061128
bin/rails db:migrate:down:main VERSION=20251006061109

MR acceptance checklist

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