gateway: bump actions/upload-artifact from 5 to 6 in /stash/save (#417)

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v5...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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

stash/save/action.yml

@@ -120,7 +120,7 @@ runs:
 
     - name: Upload Stash
       id: upload
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: ${{ steps.mung.outputs.stash_name }}
         path: ${{ inputs.path }}