build(deps): bump postcss and purgecss

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.16 and updates ancestor dependency [purgecss](https://github.com/FullHuman/purgecss). These dependencies need to be updated together.


Updates `postcss` from 7.0.27 to 8.4.16
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.27...8.4.16)

Updates `purgecss` from 2.1.2 to 5.0.0
- [Release notes](https://github.com/FullHuman/purgecss/releases)
- [Changelog](https://github.com/FullHuman/purgecss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/FullHuman/purgecss/compare/v2.1.2...v5.0.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: purgecss
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -24,7 +24,7 @@
   },
   "devDependencies": {
     "prettier": "^1.19.1",
-    "purgecss": "^2.1.2"
+    "purgecss": "^5.0.0"
   },
   "repository": {
     "type": "git",