On Go 1.21+, use cmp.Ordered instead of a private type

Go 1.21 introduces a language-maintained constraint permitting any
ordered types, use that instead of the private version defined in
sets.

Signed-off-by: Stephen Kitt <skitt@redhat.com>

Author: skitt

set/ordered.go

@@ -1,3 +1,5 @@
+//go:build !go1.21
+
 /*
 Copyright 2023 The Kubernetes Authors.
 

set/ordered_go1.21.go

@@ -0,0 +1,27 @@
+//go:build go1.21
+
+/*
+Copyright 2023 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 set
+
+import "cmp"
+
+// ordered is a constraint that permits any ordered type: any type
+// that supports the operators < <= >= >.
+// If future releases of Go add new ordered types,
+// this constraint will be modified to include them.
+type ordered = cmp.Ordered