Update github-actions to v6 (major) (#70)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/backend-ci.yml

@@ -38,7 +38,7 @@ jobs:
           java-version: 25
 
       - name: Setup Gradle
-        uses: gradle/actions/setup-gradle@v5
+        uses: gradle/actions/setup-gradle@v6
 
       - name: Check formatting
         run: ./gradlew spotlessCheck
@@ -48,15 +48,15 @@ jobs:
 
       - name: Upload coverage to Codecov
         if: always()
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v6
         with:
           files: backend/build/reports/jacoco/test/jacocoTestReport.xml
           flags: backend
           token: ${{ secrets.CODECOV_TOKEN }}
 
       - name: Upload test results to Codecov
         if: always()
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v6
         with:
           directory: backend
           flags: backend