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/runtest.go

@@ -22,25 +22,25 @@ import (
 	"github.com/ethereum/go-ethereum/cmd/devp2p/internal/v4test"
 	"github.com/ethereum/go-ethereum/internal/utesting"
 	"github.com/ethereum/go-ethereum/log"
-	"gopkg.in/urfave/cli.v1"
+	"github.com/urfave/cli/v2"
 )
 
 var (
-	testPatternFlag = cli.StringFlag{
+	testPatternFlag = &cli.StringFlag{
 		Name:  "run",
 		Usage: "Pattern of test suite(s) to run",
 	}
-	testTAPFlag = cli.BoolFlag{
+	testTAPFlag = &cli.BoolFlag{
 		Name:  "tap",
 		Usage: "Output TAP",
 	}
 	// These two are specific to the discovery tests.
-	testListen1Flag = cli.StringFlag{
+	testListen1Flag = &cli.StringFlag{
 		Name:  "listen1",
 		Usage: "IP address of the first tester",
 		Value: v4test.Listen1,
 	}
-	testListen2Flag = cli.StringFlag{
+	testListen2Flag = &cli.StringFlag{
 		Name:  "listen2",
 		Usage: "IP address of the second tester",
 		Value: v4test.Listen2,
@@ -53,7 +53,7 @@ func runTests(ctx *cli.Context, tests []utesting.Test) error {
 		tests = utesting.MatchTests(tests, ctx.String(testPatternFlag.Name))
 	}
 	// Disable logging unless explicitly enabled.
-	if !ctx.GlobalIsSet("verbosity") && !ctx.GlobalIsSet("vmodule") {
+	if !ctx.IsSet("verbosity") && !ctx.IsSet("vmodule") {
 		log.Root().SetHandler(log.DiscardHandler())
 	}
 	// Run the tests.