Revert "PostgreSQL upgrade to v18. Closes intelowlproject#3272 (intelowlproject#3292)"

This reverts commit 10f283c.

Author: mlodic

docker/env_file_postgres_template

@@ -1,4 +1,3 @@
 POSTGRES_PASSWORD=password
 POSTGRES_USER=user
-POSTGRES_DB=intel_owl_db
-PGDATA=/var/lib/postgresql/pgdata
+POSTGRES_DB=intel_owl_db

docker/postgres.override.yml

@@ -1,10 +1,10 @@
 services:
 
   postgres:
-    image: library/postgres:18-alpine
+    image: library/postgres:16-alpine
     container_name: intelowl_postgres
     volumes:
-      - postgres_data_v18:/var/lib/postgresql
+      - postgres_data:/var/lib/postgresql/data/
     env_file:
       - ./env_file_postgres
     healthcheck:
@@ -25,6 +25,3 @@ services:
     depends_on:
       postgres:
         condition: service_healthy
-
-volumes:
-  postgres_data_v18: