Pin PostCSS to resolve advisory, remove explicit dependency

[aduth]

🛠 Summary of changes

Updates PostCSS to resolve a (moderate) security advisory.

Couple notes:

• As of Switch from PostCSS/Autoprefixer/CSSNano to Parcel CSS #6591, we no longer use PostCSS directly, so it's removed as an explicit dependency
• We still have a dependency through stylelint, and while they have merged an update, there's not yet a published version containing this change, so this assigns resolutions instead. Per SemVer version ranges, this should be expected to be safe, and is a designed use-case for resolutions.

We should follow-up to remove resolutions entry once stylelint publishes an updated version.

📜 Testing Plan

1. yarn install
2. yarn lint:css