update PostgreSQL configuration in deployment files

- Change PostgreSQL volume mount from /var/lib/postgresql/data to /var/lib/postgresql
  This allows PostgreSQL to manage the data directory structure internally

- Add PostgreSQL version configuration variable to all docker-compose files:
  * POSTGRES_VERSION for deployment/nginx
  * APPSEC_POSTGRES_VERSION for all deployment/docker-compose variants

- Update PostgreSQL image tag to use version variable (set to version 18)
  Changed from 'postgres' to 'postgres:' or 'postgres:'

- Add PostgreSQL version variable to all .env files with default value of 18

deployment/docker-compose/kong-lua-plugin/docker-compose.yaml

@@ -106,14 +106,14 @@ services:
   appsec-db:
     profiles:
       - standalone
-    image: postgres
+    image: postgres:${APPSEC_POSTGRES_VERSION}
     container_name: appsec-db
     restart: unless-stopped
     environment:
       - POSTGRES_PASSWORD=${APPSEC_DB_PASSWORD}
       - POSTGRES_USER=${APPSEC_DB_USER} 
     volumes:
-      - ${APPSEC_POSTGRES_STORAGE}:/var/lib/postgresql/data
+      - ${APPSEC_POSTGRES_STORAGE}:/var/lib/postgresql
 
 ## example juice-shop backend container (vulnerable webserver, USE ONLY FOR TESTING AND IN LAB ENV)
   juiceshop-backend: