Honor gitlab.psql.ci.enabled=false when generating database.yml

What does this MR do?

This allows users to opt-out of out ci: section when the default changes to generate both main: and ci: sections in %16.0.

The default value for this will be true so no behaviour changes if there's no config change.

It should be safe to use the enabled key for this purpose as config/database.yml does not use enabled.

Related issues

#4264 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes => !3016 (comment 1308498174); Results at !3016 (comment 1312292345)
• Documentation created/updated => !2990 (merged)
• Tests added
• [-] Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened => gitlab-org/omnibus-gitlab!6688 (merged)
• [-] Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.