1.6.13 release notes (#8344)

* 1.6.13 release notes

* Fix linter issues

* Update index.md

content/en/news/releases/1.6.x/announcing-1.6.13/index.md

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+---
+title: Announcing Istio 1.6.13
+linktitle: 1.6.13
+subtitle: Patch Release
+description: Istio 1.6.13 patch release.
+publishdate: 2020-10-27
+release: 1.6.13
+aliases:
+    - /news/announcing-1.6.13
+---
+
+This release contains bug fixes to improve robustness. This release note describes what’s different between Istio 1.6.12 and Istio 1.6.13
+
+{{< relnote >}}
+
+## Changes
+
+- **Fixed** an issue where Istiod's `cacert.pem` was under the `testdata` directory
+  ([Issue #27574](https://github.com/istio/istio/issues/27574))
+
+- **Fixed** Pilot agent app probe connection leak.
+  ([Issue #27726](https://github.com/istio/istio/issues/27726))
+