Bump postcss-loader from 5.3.0 to 6.1.0 (#628)

Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 5.3.0 to 6.1.0.
- [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@v5.3.0...v6.1.0)

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

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

package.json

@@ -24,7 +24,7 @@
     "fibers": "5.0.0",
     "mini-css-extract-plugin": "1.6.0",
     "postcss": "8.3.2",
-    "postcss-loader": "5.3.0",
+    "postcss-loader": "6.1.0",
     "sass": "1.34.1",
     "sass-loader": "12.1.0",
     "stylelint": "13.13.1",