Upgrade Gradle wrapper to 9.6.1 - #48

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-java@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

The 'adopt' legacy AdoptOpenJDK distribution is deprecated and will be removed in setup-java v6. Please migrate to the 'temurin' distribution.