Hashed Storage: prevent updating to 14.x if there is any remaining unmigrated data

Based on &2320, we've deprecated legacy storage in %13.0 and are now removing it in %14.0. Due to the nature of that change, we need to prevent updating an existing installation to 14.x, if there is still unmigrated data.

In order to do that we need to add some code to the package hook and fail with a message, if the user installation still has unmigrated data.

As a way to perform this check, we could implement in the gitlab rails repository a rake task querying for any unmigrated data.