clean() doesn't remove packages from older versions of R

[hughjonesd]

Having upgraded R, run snapshot() and reinstalled all the relevant packages, I've now got packages in both renv/library/R-4.3 and renv/library/R-4.4. I ran renv::clean() expecting it to remove the packages in R-4.3 (since those versions are indeed no longer used). But it doesn't do that. Maybe it should?