ssh: remove go 1.21+ dependency on slices

Fixes golang/go#68469

Change-Id: Ieea3c444b0458d169a6ff224e59b3b815264de89
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/598775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

ssh/server_test.go

@@ -9,7 +9,7 @@ import (
 	"fmt"
 	"io"
 	"net"
-	"slices"
+	"reflect"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -294,7 +294,7 @@ func TestBannerError(t *testing.T) {
 		"banner from PublicKeyCallback",
 		"banner from KeyboardInteractiveCallback",
 	}
-	if !slices.Equal(banners, wantBanners) {
+	if !reflect.DeepEqual(banners, wantBanners) {
 		t.Errorf("got banners:\n%q\nwant banners:\n%q", banners, wantBanners)
 	}
 }