docs(release): add improved security of metrics-server note to 1.19

- I had brought attention to this issue and mentioned the lack of
  security details in the release notes in the PR for
  cc41bba
  - was requested to submit a PR for this note as well, finally got to
    pushing this up 😅
  - this is good encouragement to switch to the `metrics-server` add-on
    as it provides better security over a self-install

Author: agilgur5

docs/releases/1.19-NOTES.md

@@ -72,6 +72,7 @@ has been updated by a newer version of kOps unless it is given the `--allow-kops
 ### Addons
 
 * Metrics Server is now available as a configurable addon. Add `spec.metricsServer.enabled: true` to the cluster spec to enable.
+  * This new addon is **more secure** than installing directly via the `metrics-server` Helm chart as kOps automatically provisions and sets TLS certs, avoiding the need to use the `--kubelet-insecure-tls` flag. See [#6879](https://github.com/kubernetes/kops/issues/6879) for more details.
 
 * Cluster Autoscaler is now availalble as a configurable addon. Add `spec.clusterAutoscaler.enabled: true` to the cluster spec to enable.