fix(docs): bump node requirement to 18 (#2690)

update docs to bump node version from 14/16 to 18, which aligns with tsp
`0.50.0`

---------

Co-authored-by: Timothee Guerin <timothee.guerin@outlook.com>

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Prerequisites
 
-- Install [Node.js](https://nodejs.org/) 14 LTS or 16 LTS
+- Install [Node.js](https://nodejs.org/) 20 LTS
 - Install [Rush](https://rushjs.io/)
 
 ```bash

docs/introduction/installation.md

@@ -7,7 +7,7 @@ title: Installation
 
 ## Requirements
 
-Install [Node.js 16 LTS](https://nodejs.org/en/download/) and ensure you are able to run the `npm` command in a command prompt:
+Install [Node.js 20 LTS](https://nodejs.org/en/download/) and ensure you are able to run the `npm` command in a command prompt:
 
 ```bash
 npm --version