Increasing urgency of notes endpoint from low to medium

What does this MR do and why?

After some investigations as part of the issues gitlab-com/gl-infra/scalability#2026 (closed) and gitlab-com/gl-infra/scalability#2108 (closed), and epic gitlab-com/gl-infra&743 (closed), the endpoint Projects::NotesController#index has been identified among one of the endpoints with the highest traffic share percentage.

Given that it consistently meets the threshold at the medium urgency, we can now change it without affecting the error budget, and assign a request urgency which reflects better its relevancy.