Add v1.26.0 release of grpc-go to interop matrix (#21513)

Add v1.26.0 release of grpc-go to interop matrix

tools/interop_matrix/client_matrix.py

@@ -151,6 +151,7 @@ LANG_RELEASE_MATRIX = {
             ('v1.23.1', ReleaseInfo(runtimes=['go1.11'])),
             ('v1.24.0', ReleaseInfo(runtimes=['go1.11'])),
             ('v1.25.0', ReleaseInfo(runtimes=['go1.11'])),
+            ('v1.26.0', ReleaseInfo(runtimes=['go1.11'])),
         ]),
     'java':
     OrderedDict([