Add `dafny migrate` to support migration and reduce backwards compatibility requirements about dafny HOT 1 OPEN

Love this idea. I'd also suggest we have a standard way for a Dafny codebase to indicate what version of Dafny it works with, so we have the opportunity to give a friendly message like "It looks like you're trying to run with a newer version of Dafny, please run dafny migrate", which would then update that metadata.

That would live naturally in a dfyconfig.toml project file, but we may even want to make it a separate mechanism for now so customers don't have to switch to project files first to be protected. smithy-dafny-based projects have a project.properties file concept that does this as well as providing a way to dynamically pick the right version of the Dafny runtime in each target language: https://github.com/aws/aws-cryptographic-material-providers-library/blob/main/project.properties

from dafny.