Bump postcss-loader to 8.0.0 (#4795)

Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 7.3.4 to 8.0.0.
- [Release notes](https://github.com/webpack-contrib/postcss-loader/releases)
- [Changelog](https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/postcss-loader/compare/v7.3.4...v8.0.0)

---
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>

{{cookiecutter.project_slug}}/package.json

@@ -24,7 +24,7 @@
     "node-sass-tilde-importer": "^1.0.2",
     "pixrem": "^5.0.0",
     "postcss": "^8.3.11",
-    "postcss-loader": "^7.0.2",
+    "postcss-loader": "^8.0.0",
     "postcss-preset-env": "^9.0.0",
     "sass": "^1.43.4",
     "sass-loader": "^14.0.0",