cmd: migrate to urfave/cli/v2 (#24751)

This change updates our urfave/cli dependency to the v2 branch of the library.
There are some Go API changes in cli v2:

- Flag values can now be accessed using the methods ctx.Bool,
  ctx.Int, ctx.String, ... regardless of whether the flag is 'local' or
  'global'.

- v2 has built-in support for flag categories. Our home-grown category
  system is removed and the categories of flags are assigned as part of
  the flag definition.

For users, there is only one observable difference with cli v2: flags must now
strictly appear before regular arguments. For example, the following command is
now invalid:

   geth account import mykey.json --password file.txt

Instead, the command must be invoked as follows:

   geth account import --password file.txt mykey.json

cmd/devp2p/nodesetcmd.go

@@ -29,25 +29,25 @@ import (
 	"github.com/ethereum/go-ethereum/p2p/enr"
 	"github.com/ethereum/go-ethereum/params"
 	"github.com/ethereum/go-ethereum/rlp"
-	"gopkg.in/urfave/cli.v1"
+	"github.com/urfave/cli/v2"
 )
 
 var (
-	nodesetCommand = cli.Command{
+	nodesetCommand = &cli.Command{
 		Name:  "nodeset",
 		Usage: "Node set tools",
-		Subcommands: []cli.Command{
+		Subcommands: []*cli.Command{
 			nodesetInfoCommand,
 			nodesetFilterCommand,
 		},
 	}
-	nodesetInfoCommand = cli.Command{
+	nodesetInfoCommand = &cli.Command{
 		Name:      "info",
 		Usage:     "Shows statistics about a node set",
 		Action:    nodesetInfo,
 		ArgsUsage: "<nodes.json>",
 	}
-	nodesetFilterCommand = cli.Command{
+	nodesetFilterCommand = &cli.Command{
 		Name:      "filter",
 		Usage:     "Filters a node set",
 		Action:    nodesetFilter,