Add v1.21.0 releases of grpc-go to interop matrix

tools/interop_matrix/client_matrix.py

@@ -124,6 +124,7 @@ LANG_RELEASE_MATRIX = {
         ('v1.18.0', ReleaseInfo(runtime_subset=['go1.11'])),
         ('v1.19.0', ReleaseInfo(runtime_subset=['go1.11'])),
         ('v1.20.0', ReleaseInfo(runtime_subset=['go1.11'])),
+        ('v1.21.0', ReleaseInfo(runtime_subset=['go1.11'])),
     ]),
     'java':
     OrderedDict([