Update reference architecture docs to use Patroni

What does this MR do?

Starting from 13.7 and later PG12 is the default version for new installs and upgrades. Repmgr is being deprecated with Patroni. The MR updates reference architecture docs to use Patroni instead of repmgr using Configuring Patroni cluster docs and Postgresql template file in GitLab Environment Toolkit as a guidance.

Related issues

• Closes omnibus-gitlab#5888 (closed)
• Closes gitlab-org/quality/reference-architectures#3 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add the product tier badge accordingly.
• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.