Bump max version of bazel to 0.21

PiperOrigin-RevId: 226385826
diff --git a/configure.py b/configure.py
index 1e732db..acb92cb 100644
--- a/configure.py
+++ b/configure.py
@@ -1554,7 +1554,7 @@
   # environment variables.
   environ_cp = dict(os.environ)
 
-  check_bazel_version('0.19.0', '0.20.0')
+  check_bazel_version('0.19.0', '0.21.0')
 
   reset_tf_configure_bazelrc()