Geo - Add required extensions for secondary tracking databases

What does this MR do?

The PostgreSQL extensions page only lists required extensions for the main GitLab database gitlabhq_production. We currently don't document the required PostgreSQL extensions for a GitLab Geo secondary tracking database gitlabhq_geo_production.

Let's add this to the docs.

We can see in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/db/geo/schema.rb#L15-16 that the schema for the tracking database requires plpgsql. Additionally, we can see this was introduced in GitLab 9.0 !1197 (diffs)

Related issues

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