fix: use PostgreSQL 18+ compatible volume mount path

PostgreSQL 18 changed data directory structure to use versioned
subdirectories. Mount at /var/lib/postgresql instead of
/var/lib/postgresql/data to support all versions 14-18.

See: docker-library/postgres#1259

Author: tobyhede

tests/docker-compose.yml

@@ -12,7 +12,7 @@ services:
     volumes:
       - ./pg/postgresql.conf:/etc/postgresql/postgresql.conf
       - ./benchmark/sql/:/etc/postgresql/benchmark/sql
-      - ./pg/data-latest:/var/lib/postgresql/data
+      - ./pg/data-latest:/var/lib/postgresql
     env_file:
       - ./pg/common.env
     networks:
@@ -44,7 +44,7 @@ services:
       - ./pg/pg_hba-tls.conf:/etc/postgresql/pg_hba.conf
       - ./tls/localhost-key.pem:/etc/postgresql/localhost-key.pem
       - ./tls/localhost.pem:/etc/postgresql/localhost.pem
-      - ./pg/data-17:/var/lib/postgresql/data
+      - ./pg/data-17:/var/lib/postgresql
 
   proxy:
     image: cipherstash/proxy:latest