Allow `admin_runner` ability to change shared runners setting

What does this MR do and why?

This change allows members of a custom role with the admin_runners permission enabled to be able to change the shared_runners_setting.

#442851

MR acceptance checklist

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Screenshots or screen recordings

Before	After
Before	After

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:custom_ability_admin_runners)

2. Visit any group CI/CD settings http://127.0.0.1:3000/groups/flightjs/-/settings/ci_cd
3. Click the Runners button.
4. Click the Enable instance runners for this group button.