docs(README): update deployment notice — cyclic.app is dead

The 'Deployment Moved' notice still points users at
github-readme-activity-graph.cyclic.app as the primary deployment, but
Cyclic.sh shut down in early 2025 and that domain no longer resolves.
The Vercel deployment is currently the only reachable one. Reword the
notice to make Vercel canonical and mark the Heroku + Cyclic URLs as
historical.