Convert CLI to urfave v2

Followed the Migration Guide at https://cli.urfave.org/migrate-v1-to-v2/
The major changes not pointed out in the migration guide are:
- context.Args() no longer produces a []slice, so context.Args().Slice()
  in substitued
- All cli.Global***** are deprecated (the migration guide is somewhat
  unclear on this)

Signed-off-by: Derek Nola <derek.nola@suse.com>

Vendor in urfave cli/v2

Signed-off-by: Derek Nola <derek.nola@suse.com>

Fix NewStringSlice calls

Signed-off-by: Derek Nola <derek.nola@suse.com>

cmd/ctr/commands/namespaces/namespaces.go

@@ -27,11 +27,11 @@ import (
 	"github.com/containerd/containerd/v2/cmd/ctr/commands"
 	"github.com/containerd/errdefs"
 	"github.com/containerd/log"
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
 // Command is the cli command for managing namespaces
-var Command = cli.Command{
+var Command = &cli.Command{
 	Name:    "namespaces",
 	Aliases: []string{"namespace", "ns"},
 	Usage:   "Manage namespaces",
@@ -43,7 +43,7 @@ var Command = cli.Command{
 	},
 }
 
-var createCommand = cli.Command{
+var createCommand = &cli.Command{
 	Name:        "create",
 	Aliases:     []string{"c"},
 	Usage:       "Create a new namespace",
@@ -64,7 +64,7 @@ var createCommand = cli.Command{
 	},
 }
 
-var setLabelsCommand = cli.Command{
+var setLabelsCommand = &cli.Command{
 	Name:        "label",
 	Usage:       "Set and clear labels for a namespace",
 	ArgsUsage:   "<name> [<key>=<value>, ...]",
@@ -89,16 +89,17 @@ var setLabelsCommand = cli.Command{
 	},
 }
 
-var listCommand = cli.Command{
+var listCommand = &cli.Command{
 	Name:        "list",
 	Aliases:     []string{"ls"},
 	Usage:       "List namespaces",
 	ArgsUsage:   "[flags]",
 	Description: "list namespaces",
 	Flags: []cli.Flag{
-		cli.BoolFlag{
-			Name:  "quiet, q",
-			Usage: "Print only the namespace name",
+		&cli.BoolFlag{
+			Name:    "quiet",
+			Aliases: []string{"q"},
+			Usage:   "Print only the namespace name",
 		},
 	},
 	Action: func(context *cli.Context) error {
@@ -141,16 +142,17 @@ var listCommand = cli.Command{
 	},
 }
 
-var removeCommand = cli.Command{
+var removeCommand = &cli.Command{
 	Name:        "remove",
 	Aliases:     []string{"rm"},
 	Usage:       "Remove one or more namespaces",
 	ArgsUsage:   "<name> [<name>, ...]",
 	Description: "remove one or more namespaces. for now, the namespace must be empty",
 	Flags: []cli.Flag{
-		cli.BoolFlag{
-			Name:  "cgroup,c",
-			Usage: "Delete the namespace's cgroup",
+		&cli.BoolFlag{
+			Name:    "cgroup",
+			Aliases: []string{"c"},
+			Usage:   "Delete the namespace's cgroup",
 		},
 	},
 	Action: func(context *cli.Context) error {
@@ -163,7 +165,7 @@ var removeCommand = cli.Command{
 
 		opts := deleteOpts(context)
 		namespaces := client.NamespaceService()
-		for _, target := range context.Args() {
+		for _, target := range context.Args().Slice() {
 			if err := namespaces.Delete(ctx, target, opts...); err != nil {
 				if !errdefs.IsNotFound(err) {
 					if exitErr == nil {