Update actions/setup-java action to v6 - #25

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: gradle/actions/setup-gradle@v3.5.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

This job uses deprecated functionality from the 'gradle/gradle-build-action' action. Consult the Job Summary for more details.