fix: Enable PostgreSQL stream selection for c9s and RHEL9

c9s/RHEL9 provides PostgreSQL 13 as the default system version as a classic
RPM package. Alternative versions are provided as modular content.
So, it requires a different installation procedure.

Author: fila43

vars/CentOS_9.yml

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: MIT
+---
+# Put internal variables here with CentOS 9 specific values.
+
+__postgresql_packages: >-
+  {{ ['@postgresql:' + postgresql_version +
+       '/server'] if postgresql_version != '13' else
+       ['postgresql-server'] }}

vars/RedHat_9.yml

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: MIT
+---
+# Put internal variables here with Red Hat Enterprise Linux 9 specific values.
+
+__postgresql_packages: >-
+  {{ ['@postgresql:' + postgresql_version +
+       '/server'] if postgresql_version != '13' else
+       ['postgresql-server'] }}