Fix old tag of custom error pages used in example (#7460)

* Fix old tag of custom error pages used in example

* Move nginx-errors to k8s registry

Since the setup for the custom-error-messages was really different from
the other images that are build using cloudbuild, I changed it to "fit
in better"

* Use Go version 1.17 for custom-error-pages

Since Go >= 1.16 required the use of modules, I also initialized the module using the name k8s.io/ingress-nginx/custom-error-pages

docs/examples/customization/custom-errors/custom-default-backend.yaml

@@ -36,7 +36,7 @@ spec:
     spec:
       containers:
       - name: nginx-error-server
-        image: quay.io/kubernetes-ingress-controller/custom-error-pages-amd64:0.3
+        image: gcr.io/k8s-staging-ingress-nginx/nginx-errors:0.48.1
         ports:
         - containerPort: 8080
         # Setting the environment variable DEBUG we can see the headers sent