Fix dashboard labels dropdown

What does this MR do?

Fix the dashboard labels dropdown on the issues page by restricting the list of projects that we load labels from.

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

Why was this MR needed?

This endpoint times out on GitLab.com with a 502 response. The current behaviour is to find and display all labels in all public projects.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34590 (closed)