Stop using math/rand.Read and rand.Seed (deprecated in Go 1.20)

From golangci-lint:

> SA1019: rand.Read has been deprecated since Go 1.20 because it
>shouldn't be used: For almost all use cases, crypto/rand.Read is more
>appropriate. (staticcheck)

> SA1019: rand.Seed has been deprecated since Go 1.20 and an alternative
>has been available since Go 1.0: Programs that call Seed and then expect
>a specific sequence of results from the global random source (using
>functions such as Int) can be broken when a dependency changes how
>much it consumes from the global random source. To avoid such breakages,
>programs that need a specific result sequence should use
>NewRand(NewSource(seed)) to obtain a random generator that other
>packages cannot access. (staticcheck)

See also:

- https://pkg.go.dev/math/rand@go1.20#Read
- https://pkg.go.dev/math/rand@go1.20#Seed

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

pkg/randutil/randutil.go

@@ -0,0 +1,48 @@
+/*
+   Copyright The containerd 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 randutil provides utilities for [cyrpto/rand].
+package randutil
+
+import (
+	"crypto/rand"
+	"math"
+	"math/big"
+)
+
+// Int63n is similar to [math/rand.Int63n] but uses [crypto/rand.Reader] under the hood.
+func Int63n(n int64) int64 {
+	b, err := rand.Int(rand.Reader, big.NewInt(n))
+	if err != nil {
+		panic(err)
+	}
+	return b.Int64()
+}
+
+// Int63 is similar to [math/rand.Int63] but uses [crypto/rand.Reader] under the hood.
+func Int63() int64 {
+	return Int63n(math.MaxInt64)
+}
+
+// Intn is similar to [math/rand.Intn] but uses [crypto/rand.Reader] under the hood.
+func Intn(n int) int {
+	return int(Int63n(int64(n)))
+}
+
+// Int is similar to [math/rand.Int] but uses [crypto/rand.Reader] under the hood.
+func Int() int {
+	return int(Int63())
+}