Flink: 1.18.1 → 1.20.4

Upgrade impact: Minor upgrade. Minor releases are usually backwards-compatible, but can add or change behaviour — test in staging before production.

Fixed by upgrading to 1.20.4 iVulnerabilities that affect 1.18.1 but no longer affect 1.20.4 — the security gain from this upgrade, by exploited status then exploitation probability.

Exploited first, then by exploitation probability (EPSS).