Resolve "Expose project visibility as CI variable"

What does this MR do?

Exposes project visibility as a CI variable - CI_PROJECT_VISIBILITY

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

Any tests needed for this?

Why was this MR needed?

So that Auto DevOps can skip secret creation and usage for public projects.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• N/A API support added
• Tests added for this feature/bug
• Review
  • N/A Has been reviewed by UX
  • N/A Has been reviewed by Frontend
  • Has been reviewed by Backend
  • N/A 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?

Closes #37473 (closed)