Promote Portal2 implementation to Portal in Blueprint v5.0

[adidahiya]

We had to revert most of https://github.com/palantir/blueprint/pull/5493 because of subtle breaks caused by interactions between the new Portal implementation and the (long-deprecated) react-hot-loader library. To be safe, we're going to leave Portal as a class component in v4. This task is to track the promotion of the new "Portal2" implementation to be the standard Portal in Blueprint v5.0.