Resolve "Add PostgreSQL 11.X as a third database version"

Test plan for this to be ready

• Fresh install
  • Use PostgreSQL 10 by default
  • postgresql['version']
    • 9 works as expected
    • 11 works as expected
• Upgrade
  • From a host running PostgreSQL 9 upgrades to PostgreSQL 10
  • From a host running PostgreSQL 10 stays on PostgreSQL 10

Closes #4969 (closed)