Port remaining usage of Kotlin in generator and samples to Java - #225

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-java@v4, gradle/actions/setup-gradle@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Restore Gradle distribution 9.1.0 failed: Error: Cache service responded with 400