Updates grafana chart to 6.40.4

Author: jupyterhub-bot

mybinder/Chart.yaml

@@ -45,7 +45,7 @@ dependencies:
   #       chart v6 requires Helm 3
   #       chart v6.17.1 gives us Grafana v8.2.1
   - name: grafana
-    version: "6.37.0"
+    version: "6.40.4"
     repository: https://grafana.github.io/helm-charts
     condition: grafana.enabled