Set database_tasks to false when CI uses main database

What does this Merge Request do and why?

This prevents errors while running db:test:prepare, db:reset due to db:schema:load failing on same database.

Note database_tasks is a Rails 7 feature which we have backported in gitlab!82902 (merged). It is a no-op without the backport so it's safe to include in database.yml always

• gitlab#349589 (comment 868085024)
• gitlab#349589 (comment 865986409)

Related issue: gitlab#349589 (closed)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.