Synchronously validate foreign keys for organization_users

What does this MR do and why?

This is the final step of adding Foreign keys to organization_users table.

This MR is part of a few MR's that will replace the use of Loose Foreign Keys for organization_users table

MR	Description
!169382 (merged)	Add Foreign Key for user_id (NOT VALID)
!180028 (merged)	Add Foreign Key for organization_id (NOT VALID)
!185937 (merged)	Delete invalid organization_users records
!194587 (merged)	Finalize the 'fix invalid records' job
!199541 (merged)	Async validate the forkeign keys
This MR	Drop the NOT VALID modifier

On Gitlab.com, validation of the foreign key was successful. Using \d organization_users on postgres.ai shows this output, which is expected

Foreign-key constraints:
    "fk_8471abad75" FOREIGN KEY (organization_id) REFERENCES organizations(id) ON DELETE RESTRICT
    "fk_8d9b20725d" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE

Related to #493506 (closed)

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.