Remove unnecessary queries for .atom and .json in Dashboard::ProjectsController#index

Remove unnecessary queries for .atom and .json in Dashboard::ProjectsController#index.

Some (relevant) progress is noticeable on local tests with a good amount of data:

• Changelog entry added
• API support added
• 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 #27267 (closed)