[E2E] PROMOTE TO BLOCKING: group_webhook_events_spec.rb

What does this MR do?

Promotes the following e2e tests to the blocking bucket:

1. Manage Group WebHooks integration sends subgroup events | Testcase | Spec metrics

This MR was created based on data from reliable e2e test report: #484709 (closed)

(This MR was automatically generated by gitlab_quality-test_tooling at 2024-09-09 02:07:46 UTC)