Show warning for single database connection

Description

Show warning for single database connection. This implementation was based on the PostgreSQL version warning message.

Reference:

• https://docs.gitlab.com/ee/install/installation.html#configure-gitlab-db-settings
• https://docs.gitlab.com/ee/update/deprecations.html#single-database-connection-is-deprecated

You can test this by removing the ci: section from the config/database.yml file.

Related to #419997 (closed).