Call bulk delete runners mutation in UI

What does this MR do and why?

Implements #339525 (closed) by actually calling the backend to delete the runners. This MR depends on !93583 (merged) and !94100 (merged).

This implementation is the first MVC that we can release of a runners bulk delete:

• Each runner can be selected (but no "check all" function)
• A maximum of 50 runners can be deleted at a time: If the user selects more, the reminder runners stay in the UI, and the user can re-run the operation.
• Only available in the admin section

⚠ This feature is behind feature flag admin_runners_bulk_delete.

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag admin_runners_bulk_delete. Type rails c and then Feature.enable(:admin_runners_bulk_delete).
2. Visit Admin -> Runners http://gdk.test:3000/admin/runners
3. Confirm you can select a few runners with a checkbox on the left
4. Confirm the deletion is successful.

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.