update release script to archive the old version

[craigbox]

https://github.com/istio/istio.io/pull/16529#issuecomment-2914696351 https://github.com/istio/istio.io/pull/16529#issuecomment-2915924559

One of the things the old script did was to directly archive the existing release branch. The new script should generate PRs to do this.

(Is the old script online somewhere?)