Add v1.12.0 releases of grpc-go

tools/interop_matrix/client_matrix.py

@@ -125,6 +125,9 @@ LANG_RELEASE_MATRIX = {
         {
             'v1.11.3': None
         },
+        {
+            'v1.12.0': None
+        },
     ],
     'java': [
         {