Backport PR #11017: use correct nbformat version - fixes #11005 (#11018)

Co-authored-by: Kevin Jahns <kevin.jahns@pm.me>

packages/shared-models/src/ymodels.ts

@@ -326,7 +326,7 @@ export class YNotebook
    */
   getMetadata(): nbformat.INotebookMetadata {
     const meta = this.ymeta.get('metadata');
-    return meta ? deepCopy(meta) : { orig_nbformat: 1 };
+    return meta ? deepCopy(meta) : { orig_nbformat: 4 };
   }
 
   /**