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/puppeth/puppeth.go

@@ -24,7 +24,7 @@ import (
 	"time"
 
 	"github.com/ethereum/go-ethereum/log"
-	"gopkg.in/urfave/cli.v1"
+	"github.com/urfave/cli/v2"
 )
 
 // main is just a boring entry point to set up the CLI app.
@@ -33,11 +33,11 @@ func main() {
 	app.Name = "puppeth"
 	app.Usage = "assemble and maintain private Ethereum networks"
 	app.Flags = []cli.Flag{
-		cli.StringFlag{
+		&cli.StringFlag{
 			Name:  "network",
 			Usage: "name of the network to administer (no spaces or hyphens, please)",
 		},
-		cli.IntFlag{
+		&cli.IntFlag{
 			Name:  "loglevel",
 			Value: 3,
 			Usage: "log level to emit to the screen",