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/images/inspect.go

@@ -21,17 +21,17 @@ import (
 
 	"github.com/containerd/containerd/v2/cmd/ctr/commands"
 	"github.com/containerd/containerd/v2/pkg/display"
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
-var inspectCommand = cli.Command{
+var inspectCommand = &cli.Command{
 	Name:        "inspect",
 	Aliases:     []string{"i"},
 	Usage:       "inspect an image",
 	ArgsUsage:   "<image> [flags]",
 	Description: `Inspect an image`,
 	Flags: []cli.Flag{
-		cli.BoolFlag{
+		&cli.BoolFlag{
 			Name:  "content",
 			Usage: "Show JSON content",
 		},