Add phone_verification_enabled application setting

What does this MR do and why?

This MR introduces the new phone_verification_enabled application setting and replaces the usage of the identity_verification_phone_number feature flag in the codebase. It also updates tests associated tests to reflect the use of the new application setting to ensure the correct behavior for identity verification. This is being done because the identity_verification_phone_number feature flag has been fully rolled out, and the transition to an application setting allows for better management of these settings.

References

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• https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/work_items/884

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