#### 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
* @param {*} val The value to test
 *
 * @returns {boolean} True if value is a URLSearchParams object, otherwise false
 */
const isURLSearchParams = kindOfTest('URLSearchParams');
/**
 * Trim excess whitespace off the beginning and end of a string
 *
 * @param {String} str The String to trim
 *
 * @returns {String} The String freed of excess whitespace
 */
const trim = (str) => str.trim ?
  str.trim() : str.replace(/^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g, '');
/**
 * Iterate over an Array or an Object invoking a function for each item.
 *
 * If `obj` is an Array callback will be called passing
 * the value, index, and complete array for each item.
 *
 * If 'obj' is an Object callback will be called passing
 * the value, key, and complete object for each property.
 *
 * @param {Object|Array} obj The object to iterate
 * @param {Function} fn The callback to invoke for each item
 *
 * @param {Boolean} [allOwnKeys = false]
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 */
function forEach(obj, fn, {allOwnKeys = false} = {}) {