This commit updates the `UserPasswordExpirer.expire_user_password`
method to update `UserPassword#password_expired_at` when an existing
`UserPassword` record exists with the same `password_salt`,
`password_hash` and `password_algorithm`. This is to prevent the unique
validation error on `UserPassword#user_id` and
`UserPassword#password_hash` from being raised when the method is called
twice for a user that has not changed its password.
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