Extract namespace check out of project check

What does this MR do?

Currently check_project! also checks for namespace. We are implementing group wiki checks, which only needs namespace checks but not project checks. Therefore we move check_namespace! out of check_project! method.

Part of #207871 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.