I'd like to make sure we don't get folks stuck on v2. I know we have some internal (Google) customers who rely on the BQML stats protobuf messages (the ones in google.cloud.bigquery_v2.

I propose

1. We restore the files in google.cloud.bigquery_v2, but document that they are deprecated and are not updated.
2. We continue to return dictionaries in the relevant classes, so that all stays the same.
3. We document in the UPGRADING guide how to manually convert from the dictionaries to the classes in google.cloud.bigquery_v2 with some code samples.

Regarding the pandas connector changes, we should also document those in the UPGRADING guide, ideally with code samples on how to get / convert to the old dtypes if desired.