Enforce namespace of Kubernetes to lowercase

What does this MR do?

This MR enforces namespace of Kubernetes to lowercase

Are there points in the code the reviewer needs to double check?

Is it ok to enforce it? Or should we just improve an error message of validation?

Why was this MR needed?

This was issued at https://gitlab.com/gitlab-org/gitlab-ce/issues/34762 as Deliverable

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/34762

/cc @ayufan