bump nginx to 1.19 (mirrors core PR#4966) (#633)

Author: dgrove-oss

helm/openwhisk/values.yaml

@@ -233,7 +233,7 @@ activationStoreBackend: "CouchDB"
 # Nginx configurations
 nginx:
   imageName: "nginx"
-  imageTag: "1.17"
+  imageTag: "1.19"
   imagePullPolicy: "IfNotPresent"
   replicaCount: 1
   restartPolicy: "Always"