docs(contributing): update Node.js version to 20 to match engines (#3604)

Aligns with the root and package engines ("^20 || >=22"). Node 16 is
EOL, and the current toolchain (ESLint 9, typescript‑eslint 8, Vite 6,
Playwright ~1.49) requires ≥18.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Updates Node.js requirement in `docs/contributing.md` from 16 to 20.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
e0175f1. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: Elliot Winkler <[email protected]>

Author: KitaroMoha563

docs/contributing.md

@@ -2,7 +2,7 @@
 
 ## Getting started
 
-- Install [Node.js](https://nodejs.org) version 16.
+- Install [Node.js](https://nodejs.org) version 20.
   - If you're using [NVM](https://github.com/creationix/nvm#installation)
     (recommended), `nvm use` will ensure that the right version is installed.
 - Install [Yarn v3](https://yarnpkg.com/getting-started/install).