Default to two database connections for source install

What does this MR do and why?

Default to two database connections for source install

Update tests setup script

Related issue: #388030 (closed)

NOTE: config/database.yml.postgresql is also used by CI. The full pipeline will test all scenarios:

• Two databases (ci, main db) - e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/4225881433
• One database (main db), e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/4225881474
• One database, two connections both to main: db, e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/4225881511

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Follow the source install instructions in doc/install/installation.md using this branch

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

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