Allow admin to create an identity verification exemption

What does this MR do and why?

When an identity verification exemption is created, a user just needs to confirm an email address to confirm their identity. They do not need to confirm their identity with a phone number and/or a credit card.

This also removes the phone number exemption button from the admin interface as that has become redundant with the self-service option that was introduced with this MR.

Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/459

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable identity verification

   Feature.enable(:identity_verification)
   Feature.enable(:identity_verification_phone_number)
   ApplicationSetting.current.update(email_confirmation_setting: 2, require_admin_approval_after_user_signup: false)

2. Visit http://localhost:3000/users/sign_up, create a new account and stop when on the identity verification page (http://localhost:3000/users/identity_verification)

3. Update the new created user's risk score to Medium

   User.last.custom_attributes.create!(key: UserCustomAttribute::ARKOSE_RISK_BAND, value: Arkose::VerifyResponse::RISK_BAND_MEDIUM)

4. Refresh the page and verify the new user needs to verify their phone number

5. Open a second incognito window, login as an admin and visit the users page: http://localhost:3000/admin/users/{User.last.username}

6. As admin, click the Create identity verification exemption button

7. Refresh the first window and verify the new user does not need to verify their phone number anymore

8. As admin, click the Remove identity verification exemption button

9. Refresh the first window and verify the new user does not need to verify their phone number again

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.