Document upgrading Geo from 8.17.x to 9.1.x

It is natural to assume that this page should be followed exactly for an upgrade from 8.17.x to 9.1.x but it does not work. https://docs.gitlab.com/ee/gitlab-geo/database.html#step-3-initiate-the-replication-process

The following steps are problematic even for someone trying to upgrade to 9.0 because the link to that script is now gone with 9.1 and has been replaced by gitlab-ctl replicate-geo-database --host=1.2.3.4

Here are the steps that do not make sense without a link to the script:

9. [secondary] Create the replica.sh script as described in the database configuration document.

10. [secondary] Run the recovery script using the credentials from the previous step:

sudo bash /tmp/replica.sh

11. [secondary] Reconfigure GitLab:

sudo gitlab-ctl reconfigure

Furthermore. 9.1 now has geo_secondary_role['enable'] = true which is required for Geo to work with 9.1 and it makes geo_postgresql['enable'] = true and all the other steps to enable the tracking database irrelivant.

Since we had several bugs in Geo in 9.0 that were fixed in patch releases of 9.1 I would prefer no one upgrades Geo to 9.0 so my suggestion is to not keep any documentation for upgrading to 9.0

@axil what do you think?