Bump postcss from 7.0.32 to 8.1.4

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.32 to 8.1.4.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.32...8.1.4)

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

package.json

@@ -17,7 +17,7 @@
   },
   "homepage": "https://github.com/tbekaert/postcss-cleaner",
   "dependencies": {
-    "postcss": "^7.0.2"
+    "postcss": "^8.1.4"
   },
   "devDependencies": {
     "coveralls": "^3.0.0",