Updates test to use existing admin user

What does this MR do and why?

Due to the opt_in_identity_verification feature flag being enabled, any new users created after May 30th, 2024 are now required to do an extra step of validation to view the pipeline. This affected free_namespace_storage_spec.rb as it created a new user that attempted to view the pipeline. This change switches to simply using the existing gitlab-qa user as that was created before May 30th and it didn't need a new user for what it's verifying.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/474501

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.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up the necessary environment variables to run this against Staging.
2. Run bundle exec rspec qa/specs/features/ee/browser_ui/11_fulfillment/utilization/free_namespace_storage_spec.rb