Enable searching of projects by full path

What does this MR do?

Changes the project search query to search in routes.path instead of projects.path so that we can search by the parent group path.

This would help with #20444 (closed)

I put this behind the :project_search_by_full_path feature flag that's default enabled so that we can turn it off in case this query causes performance issues on large instances like GitLab.com

Screenshots

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.
• [-] Tested in all supported browsers