Add test cases to verify Email-based OTP does not satisfy Group and Instance 2FA Requirements

What does this MR do and why?

Add 2 test cases to verify that Email-based OTP does not satisfy Group & Instance Two-Factor Requirements

The change surfaced a flakiness in ee/spec/services/ee/search/group_service_milestones_visibility_spec.rb, so we are also modifying this test by temporarily skipping the save_namespace_details_changes callback to avoid expensive pg operation.

References

Re: https://gitlab.com/gitlab-org/gitlab/-/issues/561575

Screenshots or screen recordings

Before	After

How to set up and validate locally

bundle exec rspec spec/features/profiles/two_factor_auths_spec.rb

Make sure the test passes. The CI pipeline should be sufficient to validate.

MR acceptance checklist

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