.*: add verify script for go directive changes

Makefile

@@ -4,6 +4,12 @@ all:
 
 test:
 	go test -race ./...
-	./hack/verify-examples.sh
 	go -C v2 test -race ./...
 	(cd v2 && ./hack/verify-examples.sh)
+
+# We verify for the maximum version of the go directive as 1.20
+# here because the oldest go directive that exists on our supported
+# release branches in k/k is 1.20.
+verify:
+	./hack/verify-examples.sh
+	./hack/verify-go-directive.sh 1.20

hack/verify-go-directive.sh

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+# Copyright 2024 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.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+function usage {
+  local script="$(basename $0)"
+
+  echo >&2 "Usage: ${script} <maximum go directive>
+This script should be run at the root of a module.
+
+Compare the go directive in the local working copy's go.mod
+to the specified maximum version it can be. Versions provided
+here are of the form 1.x.y, without the 'go' prefix.
+Examples:
+  ${script} 1.20
+  ${script} 1.21.6
+"
+  exit 1
+}
+
+max="$1"
+# If max is empty, print usage and error
+if [[ -z "${max}" ]]; then
+  usage;
+fi
+
+# Don't specify the version with the go prefix, just 1.x.y will do.
+if [[ ! "${max}" =~ ^[0-9]\.[0-9]+(\.[0-9]+)?$ ]]; then
+    usage
+fi
+
+current=$(awk '/^go / {print $2;}' go.mod)
+if [[ -z "${current}" ]]; then
+  echo >&2 "FAIL: could not get value of go directive from go.mod"
+  exit 1
+fi
+
+if ! printf '%s\n' "${current}" "${max}" | sort --check=silent --version-sort; then
+    echo >&2 "FAIL: current Go directive ${current} is greater than ${max}"
+    exit 1
+fi