[java-postgres] - Correct the volume mount path to support the latest version of postgres

[ForestRealms]

Fixes PostgreSQL volume mount path inconsistency in the java-postgres template.

The volume path for the db service is updated to the standardized mount point: /var/lib/postgresql. The current configuration of mounting the volume to /var/lib/postgresql/data is now considered legacy. For latest version of PostgreSQL, mounting directly to /var/lib/postgresql is the recommended best practice, enabling the container's internal entrypoint to correctly manage the version-specific PGDATA subdirectory. This change ensures configuration alignment with the standalone postgres template (as established in PR #369) and improves long-term stability.

Verified working in a local development environment.