[CHORE] Gemini Code Assist 종료 및 CodeRabbit 전환 - #198

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v3, actions/checkout@v4, actions/setup-java@v4, gradle/gradle-build-action@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Failed to restore v8-gradle|Linux|backend-ci.yml-test[37a6259cc0c1dae299a7866489dff0bd]-b7fa5561407923badd71dba1142b64070b6615fd: Error: Cache service responded with 400