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/gen-manpages/main.go

@@ -25,7 +25,7 @@ import (
 
 	"github.com/containerd/containerd/v2/cmd/containerd/command"
 	"github.com/containerd/containerd/v2/cmd/ctr/app"
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
 func main() {