[R-sig-Debian] Documentation for installing on Ubuntu outdated

[Jeroen Ooms]

Users searching for how to install R on Ubuntu probably end up on
either of these pages:

 - https://cran.r-project.org/bin/linux/ubuntu/
 - https://cran.r-project.org/bin/linux/ubuntu/fullREADME.html

Which talk about the "current" R 4.2 release, and the latter document
also goes on talking about 4.1 packages, and even an example of
setting R_LIBS_USER for R-3.5.