Bump gradle/actions from 4 to 6 - #110

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: gradle/actions/wrapper-validation@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/actions/setup-gradle' action. Consult the Job Summary for more details.