Geo - Add geo_primary_role when configuring external DB

What does this MR do?

Our documentation for configuring Geo with an external PSQL database is currently missing the step to add roles ['geo_primary_role'] to the gitlab.rb file, as per step 7 in the https://gitlab.com/gitlab-org/gitlab/-/blob/v13.12.0-ee/doc/administration/geo/setup/database.md#step-1-configure-the-primary-server section.

Later on in the external DB doc - the geo_primary_role is mentioned, adding confusion to readers as they would not have encountered this setting at all.

Related issues

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