Start soft deprecating Node 14 and document rolling policy

[pi0]

Context: https://github.com/nuxt/framework/discussions/1234

With Nuxt 3 docs, we are recommending using the active LTS version of Node.js which is 16.x currently. 14.x is getting EOL in approx ~7 months.

A lesson learned the hard way from Nuxt 2, keep supporting older versions of Node.js finally ends up being locked in with dependencies we cannot upgrade (https://github.com/nuxt/nuxt.js/blob/dev/renovate.json) welcoming security issues and so on. And in fact, it already started popping out for Nuxt 3 and our own dependencies that are tested against 16 LTS (example: unjs/giget#17).

Whether dropping support for an EOL version of Node.js should be considered breaking change or not is a long-time discussion across Node.js community and out of this issue's context but what at least we could do is do soft deprecation by warning to encourage users to migrate to the latest LTS. And also document about this strategy that we might drop support of EOL Node.js versions eventually in semver-minor considering all major platforms remain fully supported out of the box and start warning them ahead of the time about new LTS.

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Appendix: Current Node.js releases: