Documentation for upgrading to PostgreSQL 11

Overview

In GitLab 12.8, we are bundling PostgreSQL 11 as an opt-in version. New installs and auto upgrades will default to PostgreSQL 10, but we should provide clear instructions on how to optionally upgrade to PostgreSQL 11.

Requirements

• Review the docs to find any locations that we mention supported versions of PostgreSQL and make updates to include PG11
• Add upgrade instructions for single node installs, including upgrading from 9.6 and 10
• Add upgrade instructions for HA installs, including upgrading from 9.6 and 10
• Add a documentation link to the release post gitlab-com/www-gitlab-com!40766 (merged)