Display current user's username in Identity Verification page

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/708.

Display current user's username in Identity Verification page

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Before	After

How to set up and validate locally

1. Start GDK simulating SaaS

   $ export GITLAB_SIMULATE_SAAS=1
   $ gdk start

2. Enable FFs and update required settings

   $ rails c
   > Feature.enable(:identity_verification)
   > ApplicationSetting.first.update(email_confirmation_setting: 'hard', require_admin_approval_after_user_signup: false)

3. Setup Arkose

   > ApplicationSetting.first.update(arkose_labs_public_api_key: "XXX", arkose_labs_private_api_key: "YYY", )

   Note: credentials are in 1Password under ArkoseLabs API keys (DEVELOPMENT)

4. Go to http://localhost:3000/users/sign_up, fill in the signup form, and then click Register

5. Verify that you are redirected to the Identity Verification page and that the new user's username is displayed (see screenshot above)