[webhook] allow webhooks to opt out of validating status updates

[dprotaso]

ie. serving doesn't have validation logic for the status block since we expect only our controllers update it. But our webhooks are invoked for status updates

https://github.com/knative/pkg/blob/132b1e71088b721a05f447b18e4459ffe8828169/webhook/resourcesemantics/defaulting/defaulting.go#L148-L152

[github-actions[bot]]

This issue is stale because it has been open for 90 days with no activity. It will automatically close after 30 more days of inactivity. Reopen the issue with /reopen. Mark the issue as fresh by adding the comment /remove-lifecycle stale.

[pierDipi]

/remove-lifecycle stale /triage accepted