Turn backfill off for design model IID

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In #214978 (closed) we're adding an iid to the DesignManagement::Design model and backfill it for existing rows.

In %14.0 or later, we can:

• Remove backfill: true from the call to has_internal_id.
• Change the design_management_designs.iid column to be NOT NULL.

Reference:

• https://docs.gitlab.com/omnibus/update/#mandatory-upgrade-paths-for-version-upgrades
• https://docs.gitlab.com/ee/policy/maintenance.html#upgrade-recommendations