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/main.go

@@ -24,10 +24,10 @@ import (
 	"github.com/containerd/containerd/v2/cmd/ctr/app"
 	"github.com/containerd/containerd/v2/internal/hasher"
 	"github.com/containerd/containerd/v2/pkg/seed" //nolint:staticcheck // Global math/rand seed is deprecated, but still used by external dependencies
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
-var pluginCmds = []cli.Command{}
+var pluginCmds = []*cli.Command{}
 
 func init() {
 	//nolint:staticcheck // Global math/rand seed is deprecated, but still used by external dependencies