require node 20.6+ (#579)

* require node 20.6+

* Update getting-started.md

Author: mbostock

docs/getting-started.md

@@ -1,6 +1,6 @@
 # Getting started
 
-The Observable CLI is a Node.js application<!-- and is published to npm as [`@observablehq/cli`](https://www.npmjs.com/package/@observablehq/cli)-->. As the name suggests, the CLI lives on the command line; the instructions below are intended to run in your [terminal](https://support.apple.com/guide/terminal/open-or-quit-terminal-apd5265185d-f365-44cb-8b09-71a064a42125/mac). You’ll need to install [Node.js 20 or later](https://nodejs.org/) before you can install the CLI.
+The Observable CLI is a Node.js application<!-- and is published to npm as [`@observablehq/cli`](https://www.npmjs.com/package/@observablehq/cli)-->. As the name suggests, the CLI lives on the command line; the instructions below are intended to run in your [terminal](https://support.apple.com/guide/terminal/open-or-quit-terminal-apd5265185d-f365-44cb-8b09-71a064a42125/mac). You’ll need to install [Node.js 20.6 or later](https://nodejs.org/) before you can install the CLI.
 
 ## Installing
 

package.json

@@ -100,6 +100,6 @@
     "undici": "^5.27.2"
   },
   "engines": {
-    "node": "^18 <18.19.0 || >=20"
+    "node": ">=20.6"
   }
 }