fix(deps): pin postcss override to root version, removes nested 8.4.31

Closes the postcss XSS via Unescaped </style> alert (#8). Next.js bundled
its own postcss 8.4.31 nested under node_modules/next/node_modules. The
$postcss override forces all instances to use the root version (8.5.12).

Author: dotsystemsdevs