Bump postcss-loader from 6.2.1 to 7.0.0 (#952)

Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 6.2.1 to 7.0.0.
- [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@v6.2.1...v7.0.0)

---
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>

package.json

@@ -56,7 +56,7 @@
     "markdown-it": "^13.0.1",
     "mini-css-extract-plugin": "^2.6.0",
     "postcss-less": "^6.0.0",
-    "postcss-loader": "^6.2.1",
+    "postcss-loader": "^7.0.0",
     "prettier": "^2.6.2",
     "stylelint": "^14.8.3",
     "stylelint-config-recommended": "^7.0.0",