Add external DB documentation on postgresql-evr removal

[ianballou]

We need to tell users how to upgrade their databases for the postgresql-evr extension removal. This means adding:

1. When they can remove the postgresql-evr RPM
2. How to update extension permissions to prepare for the postgresql-evr removal migration

PRs for reference:

theforeman/foreman-installer#984
theforeman/puppet-katello#502
theforeman/foreman-installer#955
#3167

[maximiliankolb]

Also relates to #3374