Execute PostgreSQL upgrade on staging

Using the template: https://ops.gitlab.net/gitlab-com/gl-infra/db-migration/-/issues/10

We would like to execute the first test migration for the PostgreSQL upgrade. The date would be at 31 of March of 2020 12:30 UTC

The maintenance time for staging should be from 30 to 40 minutes.

Please upgrade 3 nodes, we will keep 3 secondaries hosts for the rollback.