Add stale flag event to webhooks

### Is your feature request related to a problem?

Customers may want to perform actions when features become stale (e.g. notify in slack)

### Describe the solution you'd like.

Add an event to audit logs when a feature becomes stale so that customers can use that webhook.

### Describe alternatives you've considered

We could add a generic tag added / removed audit log entry, I think this could also be useful for difference scenarios although it's less explicit.

### Additional context

_No response_