#### 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/utils.js
*
 * @returns {boolean} True if value is a File, otherwise false
 */
const isFileList = kindOfTest('FileList');
/**
 * Determine if a value is a Stream
 *
 * @param {*} val The value to test
 *
 * @returns {boolean} True if value is a Stream, otherwise false
 */
const isStream = (val) => isObject(val) && isFunction(val.pipe);
/**
 * Determine if a value is a FormData
 *
 * @param {*} thing The value to test
 *
 * @returns {boolean} True if value is an FormData, otherwise false
 */
const isFormData = (thing) => {
  const pattern = '[object FormData]';
  return thing && (
    (typeof FormData === 'function' && thing instanceof FormData) ||
    toString.call(thing) === pattern ||
    (isFunction(thing.toString) && thing.toString() === pattern)
  );
}
/**
 * Determine if a value is a URLSearchParams object
 *