chore: remove obsolete monitoring compose version

[rokolslab]

This removes the obsolete top-level Docker Compose version declaration from docker-compose/docker-compose.monitoring.yml.

The change is intentionally minimal and only addresses the warning emitted during static Compose validation.

Validation:

• docker compose --env-file env.example -f docker-compose.yml -f docker-compose.monitoring.yml config
• git diff --check

Scope:

• No service, image, port, volume, environment, or documentation behavior changes.
• No containers were started.
• No install scripts were run.