Enable universe domains by default to match Go SDK behavior

[stanhu]

Due to #493, I noticed that you have to explicitly set disable_universe_domain_check if you want to automatically determine the universe domain for a Google Cloud Dedicated instance:

require 'googleauth'
creds = Google::Auth.get_application_default
creds.disable_universe_domain_check = false
creds.fetch_access_token!

This results in:

=>
{"access_token"=>
  "REDACTED TOKEN",
 "expires_in"=>3362,
 "token_type"=>"Bearer",
 "universe_domain"=>"example.universe.com"}

Otherwise, if you don't disable disable_universe_domain_check you get googleapis.com:

=>
{"access_token"=>
  "REDACTED TOKEN",
 "expires_in"=>3304,
 "token_type"=>"Bearer",
 "universe_domain"=>"googleapis.com"}

With the Go SDK, I can extract credentials with the right universe domain doing this:

package main

import (
	"cloud.google.com/go/storage"
	"context"
	"fmt"
	"golang.org/x/oauth2/google"
)

func main() {
	creds, err := google.FindDefaultCredentials(context.Background(), storage.ScopeFullControl)
	if err != nil {
	   fmt.Printf("Error reading default credentials: %w\n", err)
	   return
	}

	domain, err := creds.GetUniverseDomain()
	fmt.Printf("%s\n", domain)
}

Is the universe domain metadata endpoint considered stable now to drop this disable_universe_domain_check flag?