Add tested Istio version for clarity (#2872)

* Add tested Istio version for clarity

* Adjust to 1.4

Author: markusthoemmes

docs/install/installing-istio.md

@@ -18,7 +18,13 @@ instructions.
 You need:
 
 - A Kubernetes cluster created.
-- [`istioctl`](https://istio.io/docs/setup/install/istioctl/) (v1.5.4 or later) installed.
+- [`istioctl`](https://istio.io/docs/setup/install/istioctl/) (v1.4.10 or later) installed.
+
+## Supported Istio versions
+
+The current known-to-be-stable version of Istio tested in conjunction with Knative is **v1.4.10**.
+Versions in the 1.4 line generally be fine too.
+Versions above the 1.4 line are under test but have not stabilized yet.
 
 ## Installing Istio