Script to force-delete subgroups in E2E sandbox groups

What does this MR do and why?

This allows to force-delete subgroups in E2E top-level sandbox groups. This is currently used to clean-up staging to unblock E2E tests that are currently failing because of the number of groups, preventing any new deployments as a result.

The script requires a staging admin PAT because it seems that the /groups/:id/subgroups API endpoint timeouts with a non-admin PAT.

Usage:

scripts/api/delete_e2e_subgroups.rb --endpoint https://staging.gitlab.com/api/v4 --api-token STAGING-ADMIN-PAT --sandbox gitlab-e2e-sandbox-group-4 --max 250

How to set up and validate locally

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