chore(deps): update dependency @zendeskgarden/stylelint-config to v21 (#1683)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package-lock.json

@@ -65,7 +65,7 @@
     "@zendeskgarden/css-bedrock": "9.1.1",
     "@zendeskgarden/eslint-config": "37.0.0",
     "@zendeskgarden/scripts": "2.0.4",
-    "@zendeskgarden/stylelint-config": "20.0.0",
+    "@zendeskgarden/stylelint-config": "21.0.0",
     "@zendeskgarden/svg-icons": "7.0.0",
     "babel-plugin-module-resolver": "5.0.0",
     "babel-plugin-react-remove-properties": "0.3.0",