PostgreSQL 11 upgrade procedure for GEO installations

We need to support upgrading GEO installations from PostgreSQL 9.6.x or 10.X to PostgreSQL 11.X. Initially this will be an opt-in upgrade, but will become opt out in %12.10 and mandatory prior to GitLab 13.0. PostgreSQL 9.6 and 10 are scheduled to be removed in GitLab 13.0.

The current procedure should not need to be deviated from significantly.

We should verify the following GEO installation types

1. Single node primary and secondaries
2. Separate database nodes on primary. Secondary with separate node running postgresql and geo-postgresql.

The pg-upgrade command does not currently support running on a secondary with separate nodes for postgresql and geo-postgresql. This is a major change that will be worked on separately in #4838 (closed)