tools::R_user_dir breaks usage with older R versions

[al-obrien]

The following additions of tools::R_user_dir in the pkgdown 2.1.0 version appears to break compatibility with R versions prior to 4.0

pkgdown/R/external-deps.R

Line 94 in 1407329

cache_dir <- path(tools::R_user_dir("pkgdown", "cache"), name, version)

pkgdown/R/build-home-index.R

Line 222 in 1407329

cache_path <- dir_create(path(tools::R_user_dir("pkgdown", "cache"), "http"))

If these are needed, then perhaps the CRAN dependency reference needs adjustment, as it currently sits at >3.6?

[jayhesselberth]

Yeah, we'll either need to do one or the other (bump the required version or eliminate use of R_user_dir).

Wondering if this could also be related to the cache issues in #2696

[jayhesselberth]

Addressed in #2715