[FLINK-41226][build] Bump tcnative to 2.0.81 and run CI on Ubuntu 26.04 - #123

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: stCarolas/setup-maven@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/