add missing coredns version in cgmanifest (#4696)

Co-authored-by: CBL-Mariner Servicing Account <cblmargh@microsoft.com>

cgmanifest.json

@@ -1962,6 +1962,16 @@
         }
       }
     },
+    {
+      "component": {
+        "type": "other",
+        "other": {
+          "name": "coredns",
+          "version": "1.8.6",
+          "downloadUrl": "https://github.com/coredns/coredns/archive/v1.8.6.tar.gz"
+        }
+      }
+    },
     {
       "component": {
         "type": "other",