A non scalable solution (it's de facto not a ci/cd) is to create a meaningless commit in the repo from the Vercel git owner. I personally did it directly from the git web console just to update the project.

Main drawbacks:

• it's not a ci/cd
• it creates a lot of "garbage" commits

Main advantages:

• no need for the paid tier
• it doesn't require any setup to trigger a deployment