Add a notice that PG 12 is available

To encourage administrators to upgrade to PG12 sooner, include a message during upgrades to notify users that PG12 is now available and provide helpful messaging on how to upgrade to PG12.

Requirements

• Add a step to the upgrade process that checks the database version
• If the user is not upgrading to PG12, add a message that PG12 is now available. Include a note on how to upgrade to PG12. Note that users must migrate from repmgr to Patroni before upgrading to PG12 if they have PostgreSQL failover configured and link to the migration docs.

Issue used when adding PG11: #4992 (closed)