Bump postcss-loader from 7.3.4 to 8.1.1

Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 7.3.4 to 8.1.1.
- [Release notes](https://github.com/webpack-contrib/postcss-loader/releases)
- [Changelog](https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/postcss-loader@v7.3.4...v8.1.1)

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

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

package.json

@@ -81,7 +81,7 @@
     "postcss-custom-media": "^9.0.1",
     "postcss-custom-properties": "12.1.11",
     "postcss-import": "^15.0.1",
-    "postcss-loader": "^7.0.2",
+    "postcss-loader": "^8.1.1",
     "postcss-media-minmax": "^5.0.0",
     "postcss-nesting": "^11.2.2",
     "postcss-syntax": "^0.36.2",