Replace outdated 16.04 link with link to install options (#25875)
Replace outdated 16.04 link with link to install options (#25875)

The linked wiki page will be kept updated, so that this
README doesn't need to be updated every release.
angelapper/edx-platform,stvstnfrd/edx-platform,eduNEXT/edunext-platform,EDUlib/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,EDUlib/edx-platform,arbrandes/edx-platform,EDUlib/edx-platform,stvstnfrd/edx-platform,edx/edx-platform,eduNEXT/edx-platform,stvstnfrd/edx-platform,arbrandes/edx-platform,eduNEXT/edunext-platform,eduNEXT/edunext-platform,eduNEXT/edx-platform,edx/edx-platform,edx/edx-platform,eduNEXT/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,EDUlib/edx-platform,stvstnfrd/edx-platform,edx/edx-platform,arbrandes/edx-platform,eduNEXT/edunext-platform