There have been issues with deploying our templates to Vercel when we
rely on `engines.pnpm` configuration.
Vercel's deployments work best when we specify a `packageManager` in
`package.json` since we ship our templates without lockfiles that would
help Vercel determine the right package manager to use.
This PR adjusts the script so that it adds a `packageManager` with the
latest version of `pnpm` to our Vercel templates and removes the
`engines.pnpm` only for those variants.
b76844dac9