alternative to get_openstreetmap (source: Wickham's ggplot2 book)

Hi, I'm teaching a course using Hadley Wickham's 'ggplot2' book, and in [the section on raster maps](https://ggplot2-book.org/maps.html#rastermaps), he uses the following code:
```
mi_counties <- map_data("county", "michigan") %>%
select(lon = long, lat, group, id = subregion)

if (file.exists("mi_raster.rds")) {
mi_raster <- readRDS("mi_raster.rds")
} else {
bbox <- c(
min(mi_counties$lon), min(mi_counties$lat),
max(mi_counties$lon), max(mi_counties$lat)
)
mi_raster <- ggmap::get_openstreetmap(bbox, scale = 8735660)
saveRDS(mi_raster, "mi_raster.rds")
}
```
I'm not familiar with the subject, and was just wondering what -- if any -- appropriate alternative may exist.

Thanks.

Start by reading the raster maps example in the linked ggplot2 book section and checking the current documentation for ggmap::get_openstreetmap. Determine whether the function still has a supported replacement for the shown Michigan bounding-box workflow. Done means documenting a confirmed alternative and verifying that it can produce the raster map used by the example.

Written by the indexing model from the issue text.