Add PostgreSQL upgrade instructions on EL8

Includes a workaround for BZ 1935301 in upgrades. postgresql-upgrade is
unable to deal with data_directory in postgresql.conf. This is added by
puppetlabs/postgresql so pretty much guaranteed to be present.

_includes/manuals/2.4/3.6_upgrade.md

@@ -126,6 +126,27 @@ Next upgrade all Foreman packages:
 {% highlight bash %}
 dnf upgrade ruby\* foreman\*
 {% endhighlight %}
+
+Foreman runs best on PostgreSQL 12 but the default version on EL8 is 10. It is recommended to upgrade by [switching module streams](https://docs.fedoraproject.org/en-US/modularity/using-modules-switching-streams/).
+
+First check which version of PostgreSQL is currently installed:
+
+{% highlight bash %}
+rpm -qv postgresql-server
+{% endhighlight %}
+
+If you're already on 12 or don't have the PostgreSQL server installed, you can continue with step 3. Otherwise upgrade:
+
+{% highlight bash %}
+systemctl stop postgresql.service
+# https://bugzilla.redhat.com/1935301
+sed -i '/^data_directory =/d' /var/lib/pgsql/data/postgresql.conf
+dnf module reset postgresql
+dnf module install postgresql:12
+dnf install postgresql-upgrade
+postgresql-setup --upgrade
+systemctl start postgresql.service
+{% endhighlight %}
 </div>
 
 <div class="upgrade_os upgrade_os_debian10 upgrade_os_ubuntu1804">