[Mock API] Update website preview workflow

[widal001]

Summary

Update the website preview workflow to:

• Use the Agile Six Cloudflare account, instead of Billy's personal account
• Use wrangler versions upload instead of a deploying a new worker, so PR previews are persistent (up to 100 previous versions)

Acceptance criteria

• PR previews are published to Agile Six's Cloudflare instance
• We can remove the cleanup stage without having lingering workers in Cloudflare
• PR comment displays the most recent commit the preview was built from