build(deps-dev): bump postcss from 8.4.16 to 8.4.21

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.16 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@8.4.16...8.4.21)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -94,7 +94,7 @@
     "lit-html": "2.4.0",
     "npm-check-updates": "16.3.16",
     "plop": "3.1.1",
-    "postcss": "8.4.16",
+    "postcss": "8.4.21",
     "postcss-advanced-variables": "3.0.1",
     "postcss-cli": "10.0.0",
     "postcss-color-function": "4.1.0",