fix: treat http 403 as an updater error

Previously 403 was handled like 404 in the updater stating that
no new update is available but that behavior is deemed incorrect
in most cases so go-tuf will divert from python-tuf's behavior.

Signed-off-by: Marvin Drees <marvin.drees@9elements.com>

Author: MDr164

metadata/updater/updater.go

@@ -491,11 +491,11 @@ func (update *Updater) loadRoot() error {
 			// downloading the root metadata failed for some reason
 			var tmpErr *metadata.ErrDownloadHTTP
 			if errors.As(err, &tmpErr) {
-				if tmpErr.StatusCode != http.StatusNotFound && tmpErr.StatusCode != http.StatusForbidden {
+				if tmpErr.StatusCode != http.StatusNotFound {
 					// unexpected HTTP status code
 					return err
 				}
-				// 404/403 means current root is newest available, so we can stop the loop and move forward
+				// 404 means current root is newest available, so we can stop the loop and move forward
 				break
 			}
 			// some other error ocurred