update PostgreSQL configuration in deployment files (#365)

- 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

Co-authored-by: Nahum Perman <nahump@checkpoint.com>

deployment/nginx/.env

@@ -15,6 +15,7 @@ USER_EMAIL=user@email.com
 DB_PASSWORD=pass
 DB_USER=postgres
 DB_HOST=appsec-db
+POSTGRES_VERSION=18
 POSTGRES_STORAGE=./postgres-data
 NGINX_CONF_DIR=./nginx-proxy-config