Error in if (latest_R_version == R_version)

When using `create_app` with usual arguments and `include_R=TRUE` I get the error:

Error in if (latest_R_version == R_version) { :
argument is of length zero

I believe the error is in get_R.R on line 23:

```
latest_R_version <- readLines("https://cran.rstudio.com/bin/windows/base/", warn = F) %>%
stringr::str_extract("[1-3]\\.[0-9]+\\.[0-9]+") %>% stats::na.omit() %>% unique()
```
The regular expression should start with `[1-4]`, since R4.+ is now the current R version.