Doesn't work with React 17 or React 18

### Problem

When using React 17 or React 18, `findInstance` returns `false`. The current algorithm seems to _only_ support React 16.

### Reproduction

I've created a minimal reproduction environment on CodeSandbox [here](https://codesandbox.io/s/react-dom-instance-issue-14-mtmejg).
This uses React 17 and returns `false`. Upgrading to React 18 will have the same issue of returning `false`.
Downgrading to React 16, however, will make `findInstance` work.

I tried using this with React Testing Library as mentioned [in the docs](https://github.com/arqex/react-dom-instance#usage-with-react-testing-library), but was unfortunately unable to do so.

I was able to get something working with React `refs` instead though, but thought I'd file an issue anyway.

### Small backstory

I was trying to get this working for testing one of my libraries, [`react-signature-canvas`](https://github.com/agilgur5/react-signature-canvas), which _needs_ to use instance methods as the Canvas API is an imperative API.

Per https://github.com/agilgur5/react-signature-canvas/pull/64#pullrequestreview-876834510, Enzyme has no official support for React 17 (https://github.com/enzymejs/enzyme/issues/2429) or React 18 (https://github.com/enzymejs/enzyme/issues/2524) yet, which has created a big blocker in the community and a shift to using React Testing Library (RTL).
RTL, however, is a pretty different library (more an integration testing library than a unit testing one like Enzyme, plus many other differences), and [doesn't even support using instance methods](https://testing-library.com/docs/react-testing-library/migrate-from-enzyme#triggering-class-methods-in-tests-wrapperinstance), so this has been a pretty difficult shift in the community.
In https://github.com/agilgur5/react-signature-canvas/pull/64#issuecomment-1033256725, I found your [blog post](https://dev.to/arqex/testing-with-react-testing-library-using-component-instances-nl6) and so thought to use this library to bridge the gap therein.

I was able to use the [unofficial, partial adapter for React 17](https://github.com/wojtekmaj/enzyme-adapter-react-17) to stick with Enzyme for a bit, but there's no such workaround for [React 18](https://dev.to/wojtekmaj/enzyme-is-dead-now-what-ekl).
So I decided to try to get RTL with this library working in https://github.com/agilgur5/react-signature-canvas/issues/76#issuecomment-1126129796, and unfortunately hit upon this issue.

Fortunately, I was able to workaround this by using `refs` (see https://github.com/agilgur5/react-signature-canvas/pull/88), but thought I should file this issue nonetheless for community awareness or possible future support.