Scope system hook audit events to the organization

Part of &23936. Blocked until organization owners can manage system hooks.

Problem

EE::WebHooks::CreateService and EE::WebHooks::DestroyService audit with scope: hook.parent || current_user. System hooks have no parent, so the audit event lands on the acting user, not the organization. Updates are not audited at all.

Scope

• Scope create and destroy audit events to the organization.
• Add an audit event for updates.
• Update the audit event type definitions under config/audit_events/types.
• Check EE::WebHooks::AdminDestroyService, which uses Gitlab::Audit::InstanceScope.