Switch the default value of `secure-pod-defaults` to be more secure

This is a continuation of https://github.com/knative/serving/issues/14029

We've introduced an intermediate setting that is secure and allows root images. We didn't switch this to the default in 1.20 but will do this for 1.21 in order to give people enough notice.

Start by reading the linked continuation issue, knative/serving#14029, and locate the current `secure-pod-defaults` setting and its tests. The change is complete when the secure setting becomes the default for the 1.21 release while retaining support for root images.

Written by the indexing model from the issue text.