Deprecate the v1 API

[vishh]

The v1 API is used internally as well and it is clunky and hard to use at this point. Let's deprecate it and support only v2. To begin with, we can let users enable the v1 API on-demand for two releases and then remove the API completely in a subsequent release.

cc @timstclair

[timstclair]

xref: #1174