Bump postcss from 7.0.35 to 8.1.1

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.35 to 8.1.1.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.35...8.1.1)

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

package.json

@@ -102,7 +102,7 @@
     "object-inspect": "1.3.0",
     "osm-community-index": "2.1.1",
     "phantomjs-prebuilt": "~2.1.11",
-    "postcss": "^7.0.27",
+    "postcss": "^8.1.1",
     "postcss-selector-prepend": "^0.5.0",
     "rollup": "~2.23.0",
     "rollup-plugin-includepaths": "~0.2.3",