Allow `admin_runner` ability to change stale runner pruning setting

What does this MR do and why?

This change allows members of a custom role with the admin_runners permission enabled to be ablet o change the allow_stale_runner_pruning setting.

#442851 (closed)

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

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:custom_ability_admin_runners)

2. Login as a user that is a member of a custom role with the admin_runners permission.
3. Visit any group CI/CD settings pages such as http://127.0.0.1:3000/groups/flightjs/-/settings/ci_cd
4. Click the Enable stale runner cleanup button.