Update package.json engines to require Node pre v20

[Onatolich]

Since Node.js v21 global.navigator is an immutable property and this code in paperbits core is making it impossible to publish:
https://github.com/paperbits/paperbits-core/blob/master/src/publishing/knockoutHtmlPagePublisherPlugin.ts#L29

[github-actions]

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This scan used axe-core 4.3.2 with Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.75 Safari/537.36.