User/Project command palette search are sending incorrect scope

What does this MR do and why?

Renamed search scopes for search modal autocomplete feature. All scopes now have plural form.

• user -> users
• project -> projects
• issue -> issues

This change is done on backend as well as frontend.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Press '/' or 's' to show search modal
2. press command + k to show commands dropdown
3. select one option to search users or issues or projects
4. search any term
5. check payload in the network tap in the browser console to see the scope form in plural
6. you should also se autocomplete results showing up in the modal

Related to #455639