Fix Geo primaryApiUrl config

What does this MR do?

This MR fixes the incorrect documentation regarding where primaryApiUrl should be configured for GitLab Geo registry replication.

Problem

The current documentation states:

# If utilising Geo, and wishing to sync the container registry.
# Define this in the primary site configs only.

However, this contradicts the Omnibus documentation and the actual GitLab Geo architecture.

Solution

According to the Omnibus implementation and GitLab Geo design:

• The primaryApiUrl should be configured on secondary sites only
• Secondary sites use this URL to communicate with the primary registry
• Primary sites do not need this configuration

Related issues

• Fixes: #5948
• Where the problem was identified: #5948 (comment 2821931258)
• Related Omnibus implementation: gitlab-org/gitlab@6ba6a201
• Related chart MR that introduced the confusion: !3440 (merged)

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• [-] Tests added/updated, and test plan for scenarios not covered by automated tests.
• [-] Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.