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/events/events.go

@@ -24,14 +24,14 @@ import (
 	"github.com/containerd/containerd/v2/pkg/events"
 	"github.com/containerd/log"
 	"github.com/containerd/typeurl/v2"
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 
 	// Register grpc event types
 	_ "github.com/containerd/containerd/v2/api/events"
 )
 
 // Command is the cli command for displaying containerd events
-var Command = cli.Command{
+var Command = &cli.Command{
 	Name:    "events",
 	Aliases: []string{"event"},
 	Usage:   "Display containerd events",
@@ -42,7 +42,7 @@ var Command = cli.Command{
 		}
 		defer cancel()
 		eventsClient := client.EventService()
-		eventsCh, errCh := eventsClient.Subscribe(ctx, context.Args()...)
+		eventsCh, errCh := eventsClient.Subscribe(ctx, context.Args().Slice()...)
 		for {
 			var e *events.Envelope
 			select {