Resolve "/ci/lint should support include keyword in config file"

What does this MR do?

Moves ci/lint page under project namespace

Why was this MR needed?

ci/lint page does not support include keyword out of the box. Since local files defined on include need a project to be validated, we decided to move ci/lint under the project scope.

include keyword will only be supported on premium versions

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43603 (closed)

Port to EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5088