Undo change to minimum node version

Author: marthacryan

CONTRIBUTING.md

@@ -77,7 +77,7 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 #### Prerequisites
 
 - git
-- [node.js](https://nodejs.org/en/). We recommend using node.js v20.x.
+- [node.js](https://nodejs.org/en/). We recommend using node.js v16.x.
   Upgrading and managing node versions can be easily done using
   [`nvm`](https://github.com/creationix/nvm) or its Windows alternatives.
 - [`npm`](https://www.npmjs.com/) v7.x and up to ensure that the