Consider removing choroplethr from the list of packages

[arilamstein]

I have moved choroplethr to "maintenance mode". At the top of https://arilamstein.com/open-source/ (which choroplethr.com redirects to) I wrote:

Note: Choroplethr is currently in “maintenance mode”. I am responding to requests from CRAN for minor updates, but am not otherwise working on the project. Please contact me if you are interested in taking ownership of the project.

While there's lots of census-related choroplethr documentation out there, and certain workflows are indeed which optimized, I think it might be better to point people to other packages instead.