Migrations: A command to upgrade snapshots and backing models

[bricelam]

I don't think our current approach of keeping the model snapshots around forever is sustainable. We should consider adding a command like dotnet ef migrations recreate that reads the models (and possibly the migrations) and re-scaffolds them. This would allow us to eventually remove obsolete model building (and migrations) APIs.

We could even issue a warning during dotnet ef migrations add that they have migrations using obsolete APIs and they need to run the new command before they are removed in a future version.