`clone()` not cloning objects with custom class names

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the bug has not already been reported

### Mongoose version

6.6.x

### Node.js version

Any

### MongoDB server version

Any

### Typescript version (if applicable)

_No response_

### Description

Take a look at this line: https://github.com/Automattic/mongoose/blob/1ef7bcda4d16a253b80181253b898fadf155e1f8/lib/helpers/clone.js#L107-L109

All objects have a `valueOf()` function that return `this` by default:

```
$ nodely
Welcome to Node.js v16.17.0.
Type ".help" for more information.
> {}.valueOf
[Function: valueOf]
>
```

So `cloneObject()` never gets called. This unfortunately means that `clone()` doesn't actually clone any non-POJO objects.

Fortunately, this bug doesn't affect much because we mostly use `clone()` for POJOs. Unfortunately, this bug is very old (dates back to this commit from Jan 2011 https://github.com/Automattic/mongoose/commit/c62bb1764905b24985a5c668b83251b1a006ff94), and fixing it blows up our test suite with a bunch of stack overflows. But this is definitely worth fixing because `clone(obj) === obj` is not very intuitive.

Popped up when working on #12549

### Steps to Reproduce

```javascript
const mongoose = require('mongoose');
const clone = require('mongoose/lib/helpers/clone');

class MyClass {}
const obj = new MyClass();

console.log(obj === clone(obj)); // true?
```

### Expected Behavior

_No response_

Start in lib/helpers/clone.js at lines 107-109 and reproduce the issue with the MyClass example from the report. Inspect the existing clone-related test suite, including the stack overflows noted in the issue. Done means custom-class instances are cloned rather than returned unchanged, without introducing those recursion failures.

Written by the indexing model from the issue text.