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/oci/oci.go

@@ -19,7 +19,7 @@ package oci
 import (
 	"fmt"
 
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 
 	"github.com/containerd/containerd/v2/cmd/ctr/commands"
 	"github.com/containerd/containerd/v2/core/containers"
@@ -28,19 +28,19 @@ import (
 )
 
 // Command is the parent for all OCI related tools under 'oci'
-var Command = cli.Command{
+var Command = &cli.Command{
 	Name:  "oci",
 	Usage: "OCI tools",
-	Subcommands: []cli.Command{
+	Subcommands: []*cli.Command{
 		defaultSpecCommand,
 	},
 }
 
-var defaultSpecCommand = cli.Command{
+var defaultSpecCommand = &cli.Command{
 	Name:  "spec",
 	Usage: "See the output of the default OCI spec",
 	Flags: []cli.Flag{
-		cli.StringFlag{
+		&cli.StringFlag{
 			Name:  "platform",
 			Usage: "Platform of the spec to print (Examples: 'linux/arm64', 'windows/amd64')",
 		},