Deprecate namespaces add_on_purchase GraphQL field

What does this MR do and why?

As discussed here, we want to deprecate the addOnPurchase field in GraphQL under the root namespace field.

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 are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

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

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/489850