Update rollup-plugin-postcss requirement from ^1.6.2 to ^2.0.0

Updates the requirements on [rollup-plugin-postcss](https://github.com/egoist/rollup-plugin-postcss) to permit the latest version.
- [Release notes](https://github.com/egoist/rollup-plugin-postcss/releases)
- [Changelog](https://github.com/egoist/rollup-plugin-postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/egoist/rollup-plugin-postcss/commits/v2.0.0)

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

package.json

@@ -55,7 +55,7 @@
     "rollup-plugin-commonjs": "^9.2.0",
     "rollup-plugin-eslint": "^5.0.0",
     "rollup-plugin-node-resolve": "^4.0.0",
-    "rollup-plugin-postcss": "^1.6.2",
+    "rollup-plugin-postcss": "^2.0.0",
     "rollup-plugin-replace": "^2.1.0",
     "rollup-plugin-svgo": "^1.0.1",
     "rollup-plugin-uglify": "^6.0.0"