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/ethkey/main.go

@@ -21,7 +21,7 @@ import (
 	"os"
 
 	"github.com/ethereum/go-ethereum/internal/flags"
-	"gopkg.in/urfave/cli.v1"
+	"github.com/urfave/cli/v2"
 )
 
 const (
@@ -36,23 +36,22 @@ var app *cli.App
 
 func init() {
 	app = flags.NewApp(gitCommit, gitDate, "an Ethereum key manager")
-	app.Commands = []cli.Command{
+	app.Commands = []*cli.Command{
 		commandGenerate,
 		commandInspect,
 		commandChangePassphrase,
 		commandSignMessage,
 		commandVerifyMessage,
 	}
-	cli.CommandHelpTemplate = flags.OriginCommandHelpTemplate
 }
 
 // Commonly used command line flags.
 var (
-	passphraseFlag = cli.StringFlag{
+	passphraseFlag = &cli.StringFlag{
 		Name:  "passwordfile",
 		Usage: "the file that contains the password for the keyfile",
 	}
-	jsonFlag = cli.BoolFlag{
+	jsonFlag = &cli.BoolFlag{
 		Name:  "json",
 		Usage: "output JSON instead of human-readable format",
 	}