Update Docs: 17.0 Remove support for single database connection

See https://docs.gitlab.com/ee/update/deprecations.html#single-database-connection-is-deprecated

Soft-stop: we will not break existing installations

Reference to enable single database can be removed:

• Update lib/gitlab/database_warnings.rb to be more explicit
• Update docs for source install
• Update docs for omnibus install
• Update docs for chart install