Enable multiple database connections by default

What does this Merge Request do and why?

As part of the ci-decomposition project, we are enabling two database connections (main: and ci:) by default in GDK. We are already running with two database connections on GitLab.com

See https://docs.gitlab.com/ee/development/database/multiple_databases.html for documentation

• Enable multiple connections, but same database by default in GDK

Related issue: gitlab#349589 (closed)

Merge Request checklist

• ⚠ This change is has some important differences (https://docs.gitlab.com/ee/development/database/multiple_databases.html), but I don't expect any breakage. 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. => gitlab!83166 (merged)
• [-] gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.