Fix tests: update postgres volume mount for PostgreSQL 18 compatibility

[Copilot]

Tests have been failing since postgres:latest updated to PostgreSQL 18, which changed the container's data directory layout to use versioned subdirectories (/var/lib/postgresql/18/main) instead of the flat /var/lib/postgresql/data path. Mounting a volume at the old path causes the entrypoint to exit with code 1, preventing postgres from starting and cascading into all dependent service/test failures.

Changes

• docker-compose.yml / docker-compose-elastic-cloud.yml: Change postgres volume mount from /var/lib/postgresql/data → /var/lib/postgresql, per the PostgreSQL 18 Docker image guidance. PostgreSQL creates its own versioned subdirectory inside the mount.
• Remove deprecated version: "2.1" top-level field from both compose files.

# Before
volumes:
  - pgdata:/var/lib/postgresql/data

# After — postgres 18+ creates pgdata/18/main internally
volumes:
  - pgdata:/var/lib/postgresql

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix tests</issue_title>
<issue_description>We'd like to fix the tests because they are failing, see https://github.com/elastic/opbeans-ruby/actions/runs/22688800574/job/65778714781?pr=305</issue_description>

<agent_instructions>You are a ruby developer expert with deep knowledge of testing</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix tests #306

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[fr4nc1sc0-r4m0n]

LGTM