feat!: Use `Error#cause` in `GaxiosError`

[danielbankhead]

Modern runtimes offer Error#cause - a standard way of defining the cause of another error.

This will also improving logging in some environments that handle cause in a special way.

Documentation:

• https://nodejs.org/api/errors.html#errorcause
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause

Migration Guide

Use the cause property rather than error.

old:

const rootError = new GaxiosError().error;

new:

const rootError = new GaxiosError().cause;

Background

Dependencies (merge this PR after):

• https://github.com/googleapis/gaxios/pull/660

Fixes #572 🦕

[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml

[danielbankhead]

Note: Should merge #660 first.