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>

content/helpers.go

@@ -21,12 +21,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"math/rand"
 	"sync"
 	"time"
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/log"
+	"github.com/containerd/containerd/pkg/randutil"
 	"github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 )
@@ -123,7 +123,7 @@ func OpenWriter(ctx context.Context, cs Ingester, opts ...WriterOpt) (Writer, er
 			// error or abort. Requires asserting for an ingest manager
 
 			select {
-			case <-time.After(time.Millisecond * time.Duration(rand.Intn(retry))):
+			case <-time.After(time.Millisecond * time.Duration(randutil.Intn(retry))):
 				if retry < 2048 {
 					retry = retry << 1
 				}