chore: update minimum Node.js version to v20 (#18)

* Update minimum Node.js version to v20

Node.js 18 reached end-of-life in September 2025.

* chore: Don't pin to an actual node version in docs

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Co-authored-by: Jesse Wright <63333554+jeswr@users.noreply.github.com>

Author: melvincarvalho

docs/guides/authenticating_with_a_script.md

@@ -6,7 +6,7 @@ The approach works with any Solid server that supports the Client Credentials gr
 
 ## Prerequisites
 
-- [Node.js](https://nodejs.org/) v18 or later (for built-in `fetch`)
+-  An [LTS or current version](https://nodejs.org/en/about/previous-releases) of [Node.js](https://nodejs.org/)
 - A Solid account with a Pod on a server that supports Client Credentials
 - Basic familiarity with JavaScript