Add [DEPRECATED] label to update namespace

What does this MR do and why?

This MR adds [DEPRECATED] label to : PUT "/api/v4/namespaces/{id}" endpoint.
This endpoint should not be used to update namespace for gitlab_subscription, or associated compute minutes or storage add-ons. This have been migrated to internal endpoints: ee/lib/gitlab_subscriptions/api/internal/api.rb .

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/9771+
• https://gitlab.com/gitlab-org/gitlab/-/issues/473624+
• Add [DEPREACTED] label (!174353 - merged)

MR acceptance checklist

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Related to #473624