Run the convex version check from `prepack`, not `prepublishOnly` (#55237)

This moves the version check script in the `convex` package from `prepublishOnly` to `prepack`.

Because of the process we use to publish the package, this verification step would never run. The Devin-authored migration to Turbopack asked contributors to run the check manually, but we can simplify the whole process.

GitOrigin-RevId: 9a3457e413e5048fcc71af597e4e17d8a6c513fc