css: update browserslist requirement.

Update the minimum browser requirement from browsers released since
2012, to browsers released in the last 5 years.

This means PostCSS won't prefix CSS properties like `flex` or other
properties supported by browsers since 2017. Babel also won't transpile
Javascript syntax that has been supported by browsers since 2017.

postcss.config.js

@@ -2,9 +2,6 @@ module.exports = {
   plugins: [
     require('postcss-import'),
     require('postcss-preset-env')({
-      autoprefixer: {
-        flexbox: 'no-2009'
-      },
       stage: 3
     })
   ]