Use separate CI database by default

What does this Merge Request do and why?

This sets gitlab.rails.databases.ci.use_main_database to default to false.

See also related documentation update gitlab!85374 (closed)

Related issue: gitlab#356349 (closed)

See above issue for FAQ

Merge Request checklist

• ❌ This change is not backward compatible. If not, please include steps to communicate to our users.
  • This change is has some important differences (https://docs.gitlab.com/ee/development/database/multiple_databases.html). We will communicate this in: 🔈
    • Engineering week-in-review
    • Every channel in https://about.gitlab.com/handbook/engineering/#keeping-yourself-informed
    • #gdk channel
    • #mr-coaching
    • #core
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed. See also gitlab!85374 (closed)
• [-] gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.