Bump postcss from 8.3.4 to 8.3.6

Bumps [postcss](https://github.com/postcss/postcss) from 8.3.4 to 8.3.6.
- [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/8.3.4...8.3.6)

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

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

package.json

@@ -34,7 +34,7 @@
     "eslint": "^7.30.0",
     "eslint-config-next": "^11.0.0",
     "jest": "^26.6.3",
-    "postcss": "^8.3.4",
+    "postcss": "^8.3.6",
     "postcss-flexbugs-fixes": "^5.0.2",
     "postcss-preset-env": "^6.7.0",
     "swr": "^0.5.6",