Allow group maintainers to see group runner list

What does this MR do and why?

• Removes and enables feature flag: [Feature flag] Rollout of `maintainers_allowed_... (#421550 - closed)
• Feature issue: Allow group maintainers to view details about r... (#384179 - closed)

This change enables group maintainers to access runners list of their group and view details in a similar way as admin and auditors do today.

Maintainers are not available to directly add "New runner" in the group.

They also won't be able to "Edit" runners that don't already have access to edit.

Changelog: changed

Screenshots or screen recordings

A view like this becomes available to group maintainers:

How to set up and validate locally

1. Login (or impersonate) a user that is a Maintainer of a Group
2. Confirm that the menu item appears
   • Old navigation: CI/CD > Runners
   • New navigation: Build > Runners
3. Visit the page
4. Confirm runners can be seen, and only project runners can be edited
5. Confirm but no runner creation/registration is possible.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #421550 (closed)