multicluster: normalize nginx configmap naming (#4508)

For the Edge-20.5.6 release notes: Mention under the Helm section that the user might wanna manually remove the `nginx-configuration` configmap that is left over after this upgrade.

Signed-off-by: Mayank Shah <mayankshah1614@gmail.com>

charts/linkerd2-multicluster/templates/gateway.yaml

@@ -3,7 +3,7 @@
 apiVersion: v1
 kind: ConfigMap
 metadata:
-  name: nginx-configuration
+  name: linkerd-gateway
   annotations:
     {{.Values.createdByAnnotation}}: {{default (printf "linkerd/helm %s" .Values.linkerdVersion) .Values.cliVersion}}
   namespace: {{.Values.namespace}}
@@ -63,7 +63,7 @@ spec:
       volumes:
         - name: config
           configMap:
-            name: nginx-configuration   
+            name: linkerd-gateway   
       containers:
         - name: nginx
           readinessProbe: