ci: 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] <[email protected]>

Author: dependabot[bot]

.github/workflows/build-extensions.yml

@@ -28,7 +28,7 @@ jobs:
             -x "*.git*" -x "*.DS_Store" -x "*.md" -x "dist/*" -x "src/*"
           
       - name: Upload Chrome/Edge artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: chrome-edge-extension
           path: vouch-protocol-extension-*.zip
@@ -79,14 +79,14 @@ jobs:
           zip -r "safari-extension-xcode-v${VERSION}.zip" "Vouch Protocol"
       
       - name: Upload Safari Xcode Project
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: safari-extension-xcode
           path: safari-extension-xcode-*.zip
           retention-days: 90
 
       - name: Upload Safari Build
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         if: always()
         with:
           name: safari-extension-build