[BUG] README's Node.js prerequisite is now wrong — needs 20.9+, not 18+

[magic-peach]

Bug description
README.md's Prerequisites section (line 108) says "Node.js 18+". Since the Next.js 15→16 upgrade (merged in #1766), Next.js itself requires Node.js 20.9.0 or later — Node 18 is no longer supported by the framework. A new contributor following the README's own instructions on Node 18 would hit a confusing failure.

Steps to reproduce
Compare README.md:108 against Next.js 16's actual Node requirement (documented in Next's own release notes / next --version compatibility check).

Expected behavior
"Node.js 20.9+".

Actual behavior
Stale prerequisite from before the framework upgrade.

[github-actions]

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Architecture notes

Reframe is a static Next.js app with output: "export". Key points:

• All video processing uses FFmpeg.wasm (client-side, no server)
• headers() in next.config.ts won't work — use vercel.json instead
• not-found.tsx requires public/404.html + vercel.json routes config

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[github-actions]

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