services: databases: Don't specify a default postgresql version.

services: databases: Don't specify a default postgresql version.

Currently, if the postgresql package major version changes, this is going to
break the service upon upgrade, because PostgreSQL will reject the data files
from the differing major version of the service.

Because it's important to either keep running a particular major version, or
intentionally upgrade, I think the configuration would be better with no
default. I think this is also going to be helpful when trying to assist users
upgrading PostgreSQL.

* gnu/services/databases.scm (<postgresql-configuration>): Remove default for
postgresql.
(postgresql-service-type): Remove the default value.
* gnu/tests/databases.scm (%postgresql-os): Update accordingly.
* gnu/tests/guix.scm (%guix-data-service-os): Update accordingly.
* gnu/tests/monitoring.scm (%zabbix-os): Update accordingly.
* gnu/tests/web.scm (patchwork-os): Update accordingly.
* doc/guix.texi (PostgreSQL): Update accordingly.