Print a deprecation notice when upgrading with an external database

Summary

In GitLab 12.10, PostgreSQL 11 will become the default version for upgrades. A user can still opt to stay on PostgreSQL 9.6 or 10 during the upgrade, but if they do, we should warn them that they will need to upgrade PostgreSQL by GitLab 13.0 when PostgreSQL 11 becomes the minimum required version. #5095 (closed) covers upgrades with bundled PostgreSQL. This issue is specifically for upgrades that involved an external PostgreSQL database.

Proposal

During reconfigure on rails nodes, query the database for its version, and if less than 11 print a warning.

The warning should read: "Note that PostgreSQL 11 will become the minimum required PostgreSQL version in GitLab 13.0 (May 2020). PostgreSQL 9.6 and PostgreSQL 10 will be removed no longer be supported in GitLab 13.0. Please consider upgrading your PostgreSQL version soon."

Requirements

The notice should print when a user runs sudo apt-get update but opts out of upgrading PostgreSQL