Add go release versions to client_matrix.py
diff --git a/tools/interop_matrix/client_matrix.py b/tools/interop_matrix/client_matrix.py
index f32bdf8..0d05e6b 100644
--- a/tools/interop_matrix/client_matrix.py
+++ b/tools/interop_matrix/client_matrix.py
@@ -103,6 +103,9 @@
         {
             'v1.8.1': None
         },
+        {
+            'v1.9.1': None
+        },
     ],
     'java': [
         {