Ensure compatibility with rstan CRAN update

Some guidance from the Rstan team:

* https://discourse.mc-stan.org/t/r-package-maintainers-using-stan-please-read/24131
* https://github.com/stan-dev/rstan/blob/StanHeaders_2.26/revdep/cran.md

I'm fairly sure we're already following best practice, and prophet doesn't appear to have issues based on rstan's reverse dependency checks, but it'd be good to doubly make sure.