feat: Raise minimum supported versions

[robcresswell]

BREAKING CHANGE: Node 10 is EOL next month, so I'm taking the opportunity to batch together some breaking changes / end of support.

• Raise the minimum Node version to 12.10
• Bump the major version of the compression-webpack-plugin
• Drop the Brotli plugin in favour of Nodes native support
• Update the package.json "files" list to make the install a little smaller
• Update the README to drop the docs around earlier versions of Nuxt; I don't want to attempt to guarantee to support for them