Add register_group_runners rule to GroupPolicy

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR introduces a new :register_group_runners rule in GroupPolicy. This allows us to replace a usage where frontend is testing for the :admin_pipeline permission in order to determine whether to show the setup instructions for a group runner.

A similar MR was recently merged: !76682 (merged)

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How to set up and validate locally

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1. In rails console enable runner registration control

   Feature.enable(:runner_registration_control)

2. Visit http://gdk.test:3000/admin/application_settings/ci_cd#js-runner-settings on your GDK

   1. In the Runner registration section, disable the Members of the group can register runners checkbox

3. Visit the Settings -> CI/CD -> Runners section for a project that is the child of a group, e.g. http://gdk.test:3000/groups/h5bp/-/settings/ci_cd. The Group runners section should look like this:

   

4. Retry step 2, this time enabling the checkbox.

5. In the screen described in step 3, you should now see the following:

   

MR acceptance checklist

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

Closes #348337 (closed)