Switch the preprod TUF url

* root-signing migration to tuf-on-ci tooling means the preprod
  repository is now published to the projects GitHub Pages url
* Otherwise the functionality is the same

This change should be done
* after root-signing has switched to tuf-on-ci
  (sigstore/root-signing#1323 merge)
* before the publishing to prod is switched on
  (sigstore/root-signing#1340)

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

.github/workflows/reusable-prober.yml

@@ -33,8 +33,8 @@ on:
       tuf_preprod_repo:
         required: false
         type: string
-        default: 'https://tuf-preprod-repo-cdn.sigstore.dev'
-        description: 'TUF Repo'
+        default: 'https://sigstore.github.io/root-signing'
+        description: 'Preprod TUF Repo'
       tuf_root_path:
         required: false
         type: string