chore(deps): bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  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

@@ -42,7 +42,7 @@ jobs:
     
     - name: Upload test results
       if: always()
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: backend-test-results
         path: backend/build/reports/tests/test/
@@ -87,7 +87,7 @@ jobs:
         npm run build
     
     - name: Upload build artifacts
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: frontend-build
         path: frontend/.next/