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

@@ -102,14 +102,14 @@ jobs:
         working-directory: src/backend
 
       - name: Upload backend binary
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: backend-bin
           path: src/backend/target/release/void-eid-backend
           retention-days: 1
 
       - name: Upload stub_api artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: stub_api
           path: src/backend/target/release/stub_api
@@ -155,7 +155,7 @@ jobs:
         working-directory: src/frontend
 
       - name: Upload dist artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: frontend-dist
           path: src/frontend/dist
@@ -244,7 +244,7 @@ jobs:
           FRONTEND_URL: http://localhost:4173
 
       - name: Upload blob report
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         if: ${{ !cancelled() }}
         with:
           name: blob-report-${{ matrix.project }}
@@ -282,7 +282,7 @@ jobs:
         working-directory: src/frontend
 
       - name: Upload merged HTML report
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: playwright-report
           path: src/frontend/playwright-report