41054-Disallow creation of new Kubernetes integrations

What does this MR do?

Disallow creation of new Kubernetes integrations as proposed on https://gitlab.com/gitlab-org/gitlab-ce/issues/41054#proposal

Why was this MR needed?

To prevent disable creation of new Kubernetes integrations unless they're active or created from template

Screenshots (if relevant)

On Project integrations page for an inactive (deprecated) KubernetesService:

On Project integrations page for active (non-deprecated) KubernetesService:

On Project integrations page for any other active (non-deprecated) service:

On Admin area (service template)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16156
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #41054 (closed)