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/tasks/attach.go

@@ -21,10 +21,10 @@ import (
 	"github.com/containerd/containerd/v2/cmd/ctr/commands"
 	"github.com/containerd/containerd/v2/pkg/cio"
 	"github.com/containerd/log"
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
-var attachCommand = cli.Command{
+var attachCommand = &cli.Command{
 	Name:      "attach",
 	Usage:     "Attach to the IO of a running container",
 	ArgsUsage: "CONTAINER",
@@ -79,7 +79,7 @@ var attachCommand = cli.Command{
 			return err
 		}
 		if code != 0 {
-			return cli.NewExitError("", int(code))
+			return cli.Exit("", int(code))
 		}
 		return nil
 	},