Bump rollup-plugin-postcss from 3.1.6 to 4.0.0

Bumps [rollup-plugin-postcss](https://github.com/egoist/rollup-plugin-postcss) from 3.1.6 to 4.0.0.
- [Release notes](https://github.com/egoist/rollup-plugin-postcss/releases)
- [Changelog](https://github.com/egoist/rollup-plugin-postcss/blob/master/CHANGELOG-OLD.md)
- [Commits](egoist/rollup-plugin-postcss@v3.1.6...v4.0.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package-lock.json

@@ -113,7 +113,7 @@
         "react-svg-loader": "^3.0.3",
         "react-test-renderer": "^16.13.1",
         "rimraf": "^3.0.2",
-        "rollup-plugin-postcss": "^3.1.1",
+        "rollup-plugin-postcss": "^4.0.0",
         "rollup-plugin-styles": "^3.10.0",
         "style-loader": "^0.23.1",
         "svg-url-loader": "^6.0.0",