CI workflow actions 버전 업그레이드

- actions/checkout v2 → v4
- actions/cache v2 제거, setup-java 내장 캐시 사용
- actions/setup-java v1 → v4 (temurin 배포판)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: unam98

.github/workflows/CI.yml

@@ -18,21 +18,14 @@ jobs:
         runs-on: ubuntu-latest
         steps:
             -   name: Checkout
-                uses: actions/checkout@v2
+                uses: actions/checkout@v4
 
-            -   name: Gradle cache
-                uses: actions/cache@v2
-                with:
-                    path: |
-                        ~/.gradle/caches
-                        ~/.gradle/wrapper
-                    key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
-                    restore-keys: |
-                        ${{ runner.os }}-gradle-
             -   name: set up JDK 17
-                uses: actions/setup-java@v1
+                uses: actions/setup-java@v4
                 with:
                     java-version: 17
+                    distribution: temurin
+                    cache: gradle
 
             - name: Write compile time google-services.json file
               env: