Show warning when a group webhook is disabled

What does this MR do and why?

This make the group webhook behavior consistent with the project webhooks. When a project webhook is auto-disabled, a warning notice is shown.

We're doing the same for group webhooks here.

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag Feature.enable(:auto_disabling_web_hooks)
2. Create a group webhook with Comments for the local gitlab-org group that points to https://httpstat.us/500
3. Create 4 notes until the webhook is auto-disabled (GroupHook.last.executable? == false)
4. Visit the gitlab-group page and see the warning notice

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.