Remove old version annotations

The "Since" annotations do not make sense to keep in the public API surface documentation.

As we are heading towards a release of 2.0, this represents a "reset" in public API, and older annotations only serve to clutter the documentation with information that is not relevant to the reader.

Everything which is public at the time of the 2.0 release can be considered available "Since: 2.0" implicitly (as this is going to be the starting point of this new stable API).

It will make sense to start adding these annotations again for any added API in 2.2 and forward.