Prevent new invalid oauth_access_token records

What does this MR do and why?

• Following docs at https://docs.gitlab.com/ee/development/database/not_null_constraints.html#prevent-new-invalid-records-current-release
• We ran a background migration to ensure that there are no tokens in the DB with expires_in NULL: !107701 (merged)
• That migration ran successfully on GPRD: !107701 (comment 1241450867)
• "We first add the NOT NULL constraint with a NOT VALID parameter, which enforces consistency when new records are inserted or current records are updated."
• Default value is 2 hours
• Part 1 of #385374 (closed)

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