chore(deps): bump postcss, css-loader and stylelint

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [css-loader](https://github.com/webpack-contrib/css-loader) and [stylelint](https://github.com/stylelint/stylelint). These dependencies need to be updated together.


Updates `postcss` from 5.2.18 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/commits/8.4.31)

Updates `css-loader` from 1.0.0 to 6.8.1
- [Release notes](https://github.com/webpack-contrib/css-loader/releases)
- [Changelog](https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/css-loader/compare/v1.0.0...v6.8.1)

Updates `stylelint` from 8.4.0 to 15.11.0
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/main/CHANGELOG.md)
- [Commits](https://github.com/stylelint/stylelint/compare/8.4.0...15.11.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: css-loader
  dependency-type: direct:development
- dependency-name: stylelint
  dependency-type: direct:development
...

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

package.json

@@ -21,7 +21,7 @@
     "@babel/preset-react": "7.0.0",
     "babel-eslint": "10.0.1",
     "babel-loader": "8.0.4",
-    "css-loader": "1.0.0",
+    "css-loader": "6.8.1",
     "del": "2.0.2",
     "enzyme": "3.8.0",
     "enzyme-adapter-react-16": "1.9.1",
@@ -35,7 +35,7 @@
     "less-loader": "4.1.0",
     "lodash.merge": "4.6.1",
     "style-loader": "0.23.1",
-    "stylelint": "8.4.0",
+    "stylelint": "15.11.0",
     "stylelint-webpack-plugin": "0.10.5",
     "text-loader": "0.0.1",
     "webpack": "4.20.2"