chore: modernize Gradle build and CI - #171

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Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3, actions/setup-java@v3, gradle/gradle-build-action@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/