Allow viewing the group runners page

What does this MR do and why?

This MR is a continuation of the work started in !151825 (merged) that allows the admin_runners custom permission the ability to view the list of Runners at the Group level. This change is gated behind the custom_ability_admin_runners feature flag.

#442851

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.

Screenshots or screen recordings

Before	After
	After: Groups - Toggle Stale Runner Cleanup

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:custom_ability_admin_runners)

2. Create a role with the Admin Runners permission enabled.

3. Assign the new role to a new user

4. Login with the new user

5. Visit any group page.

6. Click on the Build menu and ensure that the Runners menu item appears.

7. Click on the Runners menu.

8. Ensure that the page loads and displays the list of runners.