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](https://github.com/actions/upload-artifact/compare/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>

.github/workflows/backend-ci.yml

@@ -82,7 +82,7 @@ jobs:
         run: cargo build --release
 
       - name: Upload binaries
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: backend-binaries
           path: |

.github/workflows/frontend-ci.yml

@@ -46,7 +46,7 @@ jobs:
         run: npm run build
 
       - name: Upload build artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: frontend-build
           path: frontend/build/