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/nginx-unified/.env

@@ -23,6 +23,7 @@ APPSEC_DB_PASSWORD=pass
 APPSEC_DB_USER=postgres
 APPSEC_DB_HOST=appsec-db
 APPSEC_POSTGRES_STORAGE=./appsec-postgres-data
+APPSEC_POSTGRES_VERSION=18
 
 ## Make sure to have a valid NGINX configuration file default.conf in the folder specified for NGINX_CONFIG.
 ## For deployment of a simple lab testing environment, you can deploy the example configuration provided