Draft: Add loose foreign keys

What does this MR do and why?

Add loose foreign keys

The code diff introduces a loose foreign key on the subscription_user_add_on_assignments table, allowing for orphaned records when the referenced user is deleted. This change is accompanied by the removal of the foreign key constraint on the users table, ensuring data integrity. Additionally, a migration is added to remove the foreign key constraint on the subscription_user_add_on_assignments table, and a test is included to verify the cleanup behavior of the loose foreign key. The diff also updates the documentation to reflect the new loose foreign key.

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Related to #444666