E2E: Enable auto_disabling_web_hooks ff for e2e test

What does this MR do and why?

Resolves #393274 (closed) and #391208 (closed)

The E2E test requires auto_disabling_web_hooks feature flag to be on. This feature flag was defaulted to false with #390157 (closed)

This MR adds code to enable the auto_disabling_web_hooks feature flag before the test.

Screenshots or screen recordings

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How to set up and validate locally

1. From the qa directory, run:

   GITLAB_QA_ADMIN_ACCESS_TOKEN="<your_admin_pat>" QA_DEBUG=true CHROME_HEADLESS=false bundle exec bin/qa Test::Instance::All http://gdk.test:3000 qa/specs/features/api/1_manage/integrations/webhook_events_spec.rb:118

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.