Add PostgreSQL upgrade instructions on EL8

_includes/manuals/2.4/3.6_upgrade.md

@@ -117,6 +117,24 @@ 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. The installer will try to use version 12 but can't upgrade existing databases. An upgrade can be performed by [switching module streams](https://docs.fedoraproject.org/en-US/modularity/using-modules-switching-streams/).
+
+First make sure you have version 10 installed:
+
+{% highlight bash %}
+rpm -qv postgresql-server
+
+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
+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">