Update kubesec.bzl to new starlark attr.label API (#2823)

* Update kubesec.bzl

* fix rule take 2

tools/kubesec.bzl

@@ -20,8 +20,7 @@ k8s_encrypted_secret = rule(
         default = "//tools:kubesec",
       ),
       "template": attr.label(
-          allow_files = True, 
-          single_file = True, 
+          allow_single_file = True, 
           mandatory = True
       ),
       "out": attr.output(mandatory = True),