Move to `postcss-load-config` 5.0

[ai]

In new version we removed Node.js 14 and 16 (they both don’t have security updates anymore) and improved ESM and TS support.

[RyanZim]

PR is filed here: #462. We'll have to drop Node 14 & 16 here as well, so it'll be a major release. May be a few days until I get this done, FYI.

[RyanZim]

Published in v11 🎆