Fix unselectable group webhooks

What does this MR do and why?

See #393481 (closed) - it is not currently possible to enable group webhooks for confidential note events or feature flag events.

This MR fixes that.

Screenshots or screen recordings

Broken behaviour video attached to the issue. Here's a screenshot after successfully creating a group hook with all triggers enabled (you can see confidential notes and feature flags in the list):

How to set up and validate locally

• Navigate to a groups webhook settings: http://gdk.test:3000/groups/flightjs/-/hooks
• Create a webhook with all triggers enabled
• Save
• Edit the hook and ensure all hooks show as enabled

MR acceptance checklist

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