Log when phone number verification exemption is created for a user

What does this MR do and why?

Ref: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/556

Add logging when a phone number verification exemption custom attribute is auto-created for a user.

This will enable us to monitor https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/556+.

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