#### 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
isURLSearchParams,
  isTypedArray,
  isFileList,
  forEach,
  merge,
  extend,
  trim,
  stripBOM,
  inherits,
  toFlatObject,
  kindOf,
  kindOfTest,
  endsWith,
  toArray,
  forEachEntry,
  matchAll,
  isHTMLForm,
  hasOwnProperty,
  hasOwnProp: hasOwnProperty, 
  reduceDescriptors,
  freezeMethods,
  toObjectSet,
  toCamelCase,
  noop,
  toFiniteNumber,
  findKey,
  global: _global,
  isContextDefined
};