docs: (#2308) update Node.js version requirement in README (#2318)

README listed Node.js v16.0.0 as the minimum, but .nvmrc specifies
v18 and all package.json engines fields require >=18. CI tests run
against Node 18, 20, and 22. Node 16 has been EOL since September
2023.

Update the system requirements to match the actual project
configuration.

Ref #2308

Co-authored-by: latenighthackathon <latenighthackathon@users.noreply.github.com>

Author: latenighthackathon

README.md

@@ -22,7 +22,7 @@ Faust.js is a toolkit for building Next.js applications for headless WordPress s
 
 ## System Requirements
 
-- Node.js v16.0.0 or newer (v16.8.0 when using Next.js 13 and v18.17 when using Next.js 14).
+- Node.js v18 or newer. npm v8 or newer.
 - MacOS, Windows (including WSL), and Linux are supported.
 
 ## Documentation