Remove usage of `sp` package

[KristinaRiemer]

Getting this note related to the ongoing deprecation of some spatial R packages:

The legacy packages maptools, rgdal, and rgeos, underpinning the sp package,
which was just loaded, will retire in October 2023.
Please refer to R-spatial evolution reports for details, especially
https://r-spatial.org/r/2023/05/15/evolution4.html.
It may be desirable to make the sf package available;
package maintainers should consider adding sf to Suggests:.
The sp package is now running under evolution status 2
     (status 2 uses the sf package in place of rgdal)

[Aariq]

Looks like maybe just leaflet is using sp, but the current version on CRAN no longer depends on it. renv::update("leaflet") might be all that needs to happen (hope for no breaking changes)