Consider deprecating the cirq-rigetti module

[mhucka]

Description of the issue

This is a proposal to deprecate the cirq-rigetti module in the next Cirq release, with intention to remove it from the code base in the release after next.

Motivation

To the best estimate we have been able to come up with, the cirq-rigetti module appears to have a total number of users in the single digits at best, yet at the same time, it has been problematic for maintenance of Cirq on more than one occasion. Given that both Quantum AI and Rigetti have limited resources for open-source software maintenance, it seems better for everyone to deprecate the module from Cirq.

Proposed process

We would aim to do roughly the following steps:

1. Include cirq-rigetti in the upcoming Cirq 1.5 release, but with an announcement that it's deprecated, meaning (roughly) that it works as-is, but we do not plan further maintenance, and it will be removed from in the following release.
2. Remove the module from the code base and documentation after the 1.5 release, and omit it from the 1.6 release later this year.