build: upgrade Flink from 1.18.1 to 1.20.5 - #248

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: project-sunbird/jira-description-action@v0.4.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/