ci(deps): Bump actions/upload-artifact from 6 to 7

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 6 to 7.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v6...v7)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -80,7 +80,7 @@ jobs:
         run: mvn -B package -DskipTests --file pom.xml
 
       - name: Upload backend artifact
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         with:
           name: backend-build
           path: target/*.jar
@@ -117,7 +117,7 @@ jobs:
         run: npm run build
 
       - name: Upload frontend build artifact
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         with:
           name: frontend-build
           path: coupon-system-frontend/dist
@@ -171,7 +171,7 @@ jobs:
         run: mvn test jacoco:report
 
       - name: Upload backend coverage report
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         with:
           name: backend-coverage-report
           path: target/site/jacoco/
@@ -213,7 +213,7 @@ jobs:
 
       - name: Upload frontend coverage report
         if: always()
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         with:
           name: frontend-coverage-report
           path: coupon-system-frontend/coverage/
@@ -266,7 +266,7 @@ jobs:
           NVD_API_KEY: ${{ secrets.NVD_API_KEY }}
 
       - name: Upload OWASP report
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         if: always()
         with:
           name: owasp-report