[FIX] 마지막 타임박스 사회자 이탈 시 종료 알림 발송 및 공유 기록 정리 구조 개선 - #279

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@v4, madrapps/jacoco-report@v1.6.1, mikepenz/action-junit-report@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

setup-java v4 is deprecated and will no longer receive updates. Please migrate to actions/setup-java@v5.