Add package_type attribute to the migration guide

Conan V2 offers a new attribute which helps Conan client to know about the package type, however, it's only documented on 2.0 branch: https://docs.conan.io/en/2.0/reference/conanfile/attributes.html#package-type

It would be documenting it on the migration guide, so people could add it always. That new attribute has no side-effects so far on Conan 1.x.