Update testing-supported-clusters.md (#2887)

Add AKS 1.31 to CMX versions.  Merge after replicatedhq/reliability-matrix#1145 is released.

Author: xavpaice

docs/vendor/testing-supported-clusters.md

@@ -402,7 +402,7 @@ Compatibility Matrix supports creating [Azure AKS](https://azure.microsoft.com/e
   </tr>
   <tr>
     <th>Supported Kubernetes Versions</th>
-    <td>1.28, 1.29, 1.30</td>
+    <td>1.28, 1.29, 1.30, 1.31</td>
   </tr>
   <tr>
     <th>Supported Instance Types</th>