fix: 🏷️ Ensure username is returned by `get_username` as a string (#7457)

In order to maintain consistency and compatibility, explicitly convert the 'username' variable to a string using the str() function before returning it. This ensures that the function consistently provides a string representation of the username, addressing potential variations in the original data type.

https://github.com/kubeflow/kubeflow/issues/7456

Update authn.py

Update notebook_template.yaml

components/crud-web-apps/jupyter/backend/apps/common/yaml/notebook_template.yaml

@@ -7,7 +7,7 @@ metadata:
     app: {name}
   annotations:
     notebooks.kubeflow.org/server-type: ""
-    notebooks.kubeflow.org/creator: {creator}
+    notebooks.kubeflow.org/creator: "{creator}"
 spec:
   template:
     spec: