`dagger ws migrate` leaves legacy SDK fields in `dagger.toml`. Convert them to `sdks`, warn about unsupported fields, and preserve unrelated text. Stop on conflicts.

```sh
dagger ws migrate --no-apply  # Preview changes
dagger ws migrate -y         # Apply changes
```

Signed-off-by: Solomon Hykes <solomon@dagger.io>
Signed-off-by: Tibor Vass <teabee89@gmail.com>
Signed-off-by: Yves Brissaud <yves@dagger.io>
Co-authored-by: Tibor Vass <teabee89@gmail.com>
Co-authored-by: Yves Brissaud <yves@dagger.io>