Upgrade minimal Node.js version to Node 12

[ehmicky]

See background at https://github.com/netlify/team-dev/issues/34

We should upgrade the minimal Node.js version to Node >=12.20.0 and make a major release.
This applies to all packages of this monorepo, including @netlify/build and @netlify/config.

This should only be done after Node 10 support in Netlify CLI has been dropped first (netlify/cli#3512).

This would not break sites because:

• The buildbot is already running @netlify/build with Node 12.
• Users would still be allowed to run Node 10 with their build commands.
• Plugins installed in the UI or in netlify.toml (not package.json) are already running with Node 12
• Local plugins and plugins in package.json are also using Node 12, except for sites with a user-defined Node.js version. For those, the user-defined Node.js version is used instead, except when it is older than the minimal Node.js version supported by @netlify/build. This means any site with such plugins and a user-defined Node.js version of 10 would run those plugins with Node 12 instead.

[ehmicky]

Started at #3873

[ehmicky]

Cleanup PRs at #3880, #3881, #3882, #3883