Consolidate Geo upgrade documentation
ZD: https://gitlab.zendesk.com/agent/tickets/83311
Customer pointed out that we have two sources of upgrade documentation:
- https://docs.gitlab.com/ee/gitlab-geo/configuration.html#upgrading-geo
- https://docs.gitlab.com/ee/gitlab-geo/updating_the_geo_nodes.html
/cc: @nick.thomas, @brodock