#### Describe the bug

if the `httpsAgent` property is set, it is not excluded from `error.toJSON()` resulting in a circular reference error

```none
TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'httpsAgent' -> object with constructor 'Agent'
    |     property 'sockets' -> object with constructor 'Object'
    |     ...
    |     property 'errored' -> object with constructor 'Object'
    --- property 'config' closes the circle
    at JSON.stringify (<anonymous>)
```

#### To Reproduce

```js
const agent = new https.Agent({  
  rejectUnauthorized: false
});

axios.get("some-dodgy-url", {
  httpsAgent: agent
}).catch((err) => {
  console.error(err.toJSON())
})
```

#### Expected behavior

The error object should be serialisable as JSON without throwing circular reference errors.

#### Environment
 - Axios 0.26.0

#### Additional context/Screenshots

See [this StackOverflow post](https://stackoverflow.com/q/71150003/283366)
lib/core/AxiosError.js
'use strict';
import utils from '../utils.js';
/**
 * Create an Error with the specified message, config, error code, request and response.
 *
 * @param {string} message The error message.
 * @param {string} [code] The error code (for example, 'ECONNABORTED').
 * @param {Object} [config] The config.
 * @param {Object} [request] The request.
 * @param {Object} [response] The response.
 *
 * @returns {Error} The created error.
 */
function AxiosError(message, code, config, request, response) {
  Error.call(this);
  if (Error.captureStackTrace) {
    Error.captureStackTrace(this, this.constructor);
  } else {
    this.stack = (new Error()).stack;
  }
  this.message = message;
  this.name = 'AxiosError';
  code && (this.code = code);