docs: update Node.js version requirement to >=20 in contributing guide

[ZeinebMegaadi]

Describe your changes

Updated the Node.js version requirement in CONTRIBUTING.md from v16+ to v20+ to match the actual requirement specified in server/package.json ("node": ">=20"). This prevents new contributors from setting up with an unsupported Node.js version and hitting cryptic errors.

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No issue was opened for this change , it's a single-word documentation fix that doesn't require one.

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const { t } = useTranslation();
<div>{t('add')}</div>

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