chore(deps): update emqx/emqx docker tag to v5.7 (#637)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

argocd/manifests/mqtt/values.yaml

@@ -8,7 +8,7 @@ image:
   repository: emqx/emqx
   pullPolicy: IfNotPresent
   # Overrides the image tag whose default is the chart appVersion.
-  tag: "5.6"
+  tag: "5.7"
   ## Optionally specify an array of imagePullSecrets.
   ## Secrets must be manually created in the namespace.
   ## ref: https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/