Updates prometheus chart to 15.16.0

Author: jupyterhub-bot

mybinder/Chart.yaml

@@ -31,7 +31,7 @@ dependencies:
   #       chart v11.16.9 maps to app v2.21.0  - 2020-12-06
   #       chart v12 requires Helm 3
   - name: prometheus
-    version: "15.15.0"
+    version: "15.16.0"
     repository: https://prometheus-community.github.io/helm-charts
     condition: prometheus.enabled