Add enum to graphql mutation

What does this MR do and why?

Addressing issue.

We usually prefer to use enum in graphql mutations, so this is just a quick refactor to apply enum instead of string.

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

screenshots to update an instance of Ci::Catalog::VerifiedNamespace

When instance is not Gitlab.com 👇 When passed in namespace is not a root namespace 👇

When namespace is unknown/invalid 👇

Success update for existing Ci::Catalog::VerifiedNamespace 👇

screenshots for new instance of Ci::Catalog::VerifiedNamespace

Before calling the endpoint 👇 When non root namespace is passed in 👇 When invalid namespace path is passed in 👇 Success creation 👇

How to set up and validate locally

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