docs: fix default Postgres version from 17 to 18

Example configs in v4.1.0 use postgresai/extended-postgres:18-0.6.2
as the default Docker image.

https://claude.ai/code/session_011sPDgBjzL2N2X6jiYyoTjQ

Author: claude

docs/database-lab/supported-databases.md

@@ -14,7 +14,7 @@ Currently, DBLab Engine fully supports the following [PostgreSQL major versions]
 - 17 (released: 2024-09-26; EOL: 2029-11-08)
 - 18 (released: 2025-09-25; EOL: 2030-11-13)
 
-By default, version 17 is used: `postgresai/extended-postgres:17`.
+By default, version 18 is used in the example configurations: `postgresai/extended-postgres:18`.
 
 The images are published in [Docker Hub](https://hub.docker.com/r/postgresai/extended-postgres).