#### 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
'ECONNABORTED',
  'ETIMEDOUT',
  'ERR_NETWORK',
  'ERR_FR_TOO_MANY_REDIRECTS',
  'ERR_DEPRECATED',
  'ERR_BAD_RESPONSE',
  'ERR_BAD_REQUEST',
  'ERR_CANCELED',
  'ERR_NOT_SUPPORT',
  'ERR_INVALID_URL'
].forEach(code => {
  descriptors[code] = {value: code};
});
Object.defineProperties(AxiosError, descriptors);
Object.defineProperty(prototype, 'isAxiosError', {value: true});
AxiosError.from = (error, code, config, request, response, customProps) => {
  const axiosError = Object.create(prototype);
  utils.toFlatObject(error, axiosError, function filter(obj) {
    return obj !== Error.prototype;
  }, prop => {
    return prop !== 'isAxiosError';
  });
  AxiosError.call(axiosError, error.message, code, config, request, response);
  axiosError.cause = error;
  axiosError.name = error.name;
  customProps && Object.assign(axiosError, customProps);
  return axiosError;
};
export default AxiosError;