rversions now uses R-hub's R version API.
The r_oldrel(), r_release(), r_release_macos(), r_release_tarball(),
and r_release_win() functions are now superseded by the new resolve()
function, see their manual pages.

• New function resolve() to resolve an R version specification.

• New function available() to list available R builds for a platform.

• New function linux_distros() to list Linux distributions supported
  by Posit or R-hub.

• New function rtools_versions() to list Rtools versions.

• New utility function current_r_platform() to look up the current
  platform, including the distribution and release on Linux.