Bump @primer/css from 21.2.2 to 21.3.1 in /demo

Bumps [@primer/css](https://github.com/primer/css) from 21.2.2 to 21.3.1.
- [Release notes](https://github.com/primer/css/releases)
- [Changelog](https://github.com/primer/css/blob/main/CHANGELOG.md)
- [Commits](primer/css@v21.2.2...v21.3.1)

---
updated-dependencies:
- dependency-name: "@primer/css"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

demo/package.json

@@ -3,7 +3,7 @@
   "private": true,
   "version": "0.1.0",
   "dependencies": {
-    "@primer/css": "^21.2.2",
+    "@primer/css": "^21.3.1",
     "@primer/primitives": "^8.1.0",
     "@rails/actioncable": "^7.1.3",
     "@rails/ujs": "^7.1.3",