fix(gatekeeper): replace deprecated exception for mattermost

chart/templates/gatekeeper/values.yaml

@@ -29,9 +29,9 @@ violations:  # Try to keep this in alpha order to make it easier to find keys
         - registry1.dso.mil
         - registry.dso.mil
       {{- if .Values.addons.mattermost.enabled }}
-      exemptContainers:
+      excludedResources:
         # Mattermost needs postgres:13 image and cannot override the upstream
-        - init-check-database
+        - mattermost/init-check-database
       {{- end }}
 
   {{- if .Values.monitoring.enabled}}