Bump postcss from 8.4.20 to 8.4.21

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.20 to 8.4.21.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.20...8.4.21)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

package-lock.json

@@ -31,7 +31,7 @@
     "css-minimizer-webpack-plugin": "4.2.2",
     "html-minifier": "4.0.0",
     "mini-css-extract-plugin": "2.7.2",
-    "postcss": "8.4.20",
+    "postcss": "8.4.21",
     "postcss-loader": "7.0.1",
     "pug": "3.0.2",
     "sass": "1.57.1",