RStan & expiring deprecations

Transferring the discussion from the forums for visibility.

We're approaching the expiration of deprecated syntax (e.g., arrays) before rstan is compatible with the new syntax and downstream packages can update. The current compromise is to delay the deprecations by a release cycle (four months), with the goal of having 2.26 (and then quickly 2.31/2.32) on CRAN in the near future so that downstream dependencies can begin to update.

We will also have the lee-way of an additional release cycle after the deprecations expire through the use of the stanc auto-formatter. But as mentioned, this will only work for one release cycle after the deprecations expire.

I personally think that this feasible, and am happy to handle most of the PRs patching downstream dependencies. But I'm not familiar enough with rstan's previous/current CRAN difficulties to know whether it's likely to get 2.26+ to CRAN soon, so feedback is appreciated

Read the linked forum discussion and rstantools issue 113 first; identify the release and deprecation dependencies and downstream packages mentioned. Done is not defined in this issue, so confirm the target release plan and concrete PR scope before starting.