Ensure identity_verification saas feature is available

What does this MR do and why?

Implements https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/work_items/654.

This MR updates Users::IdentityVerificationController to ensure identity_verification SaaS feature is available before a user can access Opt-in Identity Verification feature.

Ensure identity_verification saas feature is available

identity_verification SaaS feature should be available before a user can access Opt-in Identity Verification feature.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Start GDK without simulating SaaS

   $ gdk start

2. Enable FFs

   $ rails c
   > Feature.enable(:opt_in_identity_verification)

3. Login with root user
4. Go to http://localhost:3000/-/identity_verification
5. Validate that 404 page is shown
6. Restart GDK with SaaS simulation

   $ export GITLAB_SIMULATE_SAAS=1
   $ gdk restart

5. Refresh http://localhost:3000/-/identity_verification
6. Validate that you can see the Opt-in Identity Verification page

<!--
Example below:

1. In rails console enable the experiment fully
 ```ruby
 Feature.enable(:member_areas_of_focus)

1. Visit any group or project member pages such as http://127.0.0.1:3000/groups/flightjs/-/group_members
2. Click the invite members button. -->