Geo: Remove upgrade instructions of very old versions
When looking at https://docs.gitlab.com/ee/administration/geo/replication/updating_the_geo_nodes.html#upgrading-to-gitlab-108 and following. The ~Geo upgrade doc is cluttered with instructions of versions that are over a year old (10.8 is released on 2018-05-22).
We should see if we can remove these instructions, since we have versioned docs now and people cannot skip a major release (https://docs.gitlab.com/ee/policy/maintenance.html#upgrade-recommendations). So we can add a small note and link to https://docs.gitlab.com/11.11/ee/administration/geo/replication/updating_the_geo_nodes.html#upgrading-to-gitlab-108 instead.