Add top-level /merge_requests API endpoint

What does this MR do?

Makes it possible to query for merge requests outside the scope of a project.

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 #28422 (closed).

Related to !13004 (merged).

marked the checklist item Changelog entry added, if necessary as completed

marked the checklist item Documentation created/updated as completed

marked the checklist item API support added as completed

marked the checklist item Added for this feature/bug as completed

marked the checklist item Conform by the merge request performance guides as completed

marked the checklist item Conform by the style guides as completed

marked the checklist item Branch has no merge conflicts with master (if it does - rebase it please) as completed

added 1 commit

• 26cdd8d7 - Add top-level /merge_requests API endpoint

Compare with previous version

added 1 commit

• 26cdd8d7 - Add top-level /merge_requests API endpoint

Compare with previous version

@jameslopez This one is somewhat similar to !13004 (merged), so I hope you don't mind me assigning this one to you to?

assigned to @jameslopez

changed milestone to %9.5

added Plan [DEPRECATED] ~13884 labels

@to1ne thanks! LGTM, just left a few minor questions.

assigned to @to1ne