Enable universe domains by default to match Go SDK behavior

Due to https://github.com/googleapis/google-auth-library-ruby/pull/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:

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

This results in:

```ruby
=>
{"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`:

```ruby
=>
{"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:

```go
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?