Update k8s dependency version to 1.21

Signed-off-by: xinyanwu409 <wxinyan@vmware>

vendor/sigs.k8s.io/structured-merge-diff/v4/schema/doc.go

@@ -0,0 +1,28 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package schema defines a targeted schema language which allows one to
+// represent all the schema information necessary to perform "structured"
+// merges and diffs.
+//
+// Due to the targeted nature of the data model, the schema language can fit in
+// just a few hundred lines of go code, making it much more understandable and
+// concise than e.g. OpenAPI.
+//
+// This schema was derived by observing the API objects used by Kubernetes, and
+// formalizing a model which allows certain operations ("apply") to be more
+// well defined. It is currently missing one feature: one-of ("unions").
+package schema