Bump @rails/actioncable from 7.2.100 to 8.0.100 in /demo

Bumps [@rails/actioncable](https://github.com/rails/rails) from 7.2.100 to 8.0.100.
- [Release notes](https://github.com/rails/rails/releases)
- [Commits](https://github.com/rails/rails/commits)

---
updated-dependencies:
- dependency-name: "@rails/actioncable"
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

demo/package-lock.json

@@ -5,7 +5,7 @@
   "dependencies": {
     "@primer/css": "^21.5.1",
     "@primer/primitives": "^10.3.4",
-    "@rails/actioncable": "^7.2.100",
+    "@rails/actioncable": "^8.0.100",
     "@rails/ujs": "^7.1.501",
     "turbolinks": "^5.2.0"
   },