Upgrade minimum Node version to 14 in netlify-plugin-next

[ericapisani]

Currently the "engines" property in the plugin's package.json is set to node v12.

Given node 12 was EOLed recently, this should be upgraded to 14.

[ascorbic]

For frameworks plugins we've been matching the minimum versions in the frameworks themselves. Last time I checked, Next.js still supports 12, but hopefully that will change soon. Gatsby moved to 14 last year, which is why we can require that.

[charkour]

With Next v13, the minimum node version is v14: Docs

[taty2010]

Will need to hold off on updating since Next v12 still has node 12 as the minimum.