Get global retention policy

{  "policy": {    "artifact_ttl_days": 0,    "cleanup_interval_secs": 0,    "cleanup_target": "artifacts_only",    "dry_run": true,    "enabled": true,    "keep_statuses": [      "string"    ],    "max_age_days": 0,    "max_artifact_size_bytes": 0,    "max_builds_per_project": 0,    "updated_at": 0  }}