[source install] Default to two connections (main, and ci)

Default to two DB connections (main, and ci)

Proposal

1. Print out a warning that having only main: is deprecated if config only has main, and no ci
2. Change config/database.yml.postgresql to add ci:. This is referred to in source installation instructions
3. For other installation methods, we have:

• OmniBus : omnibus-gitlab#7451 (closed)
• Helm Charts: gitlab-org/charts/gitlab#4248 (closed)
• Docker : #391520 (closed)
• GitLab Operator : gitlab-org/cloud-native/gitlab-operator#1058 (closed)

Documentation Update

This issue should also update the documentation regarding:

1. What is changing in this release %16.0 with defaulting to two connections, as opposed to using one connection before.
2. Intructions on how to switch back to one connection. Opposite of the existing documentation that has been done prior to %16.0 for the related platform.
3. Intructions on how to instruct Omnibus/chart to keep using single connection before upgrading to %16.0 (aka pin)

Notes

1. We deliberately make this change in a major version %16.0 because it is a big change.
2. Customers can flip it back to false (e.g. if they encounter issues).