Bump actions/upload-artifact from 4 to 6 (#5233)

Bumps
[actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.

- vLLM version: release/v0.13.0
- vLLM main:
ad32e3e19c

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/_pr_image_build.yaml

@@ -86,7 +86,7 @@ jobs:
         digest="${{ steps.build.outputs.digest }}"
         touch "${{ runner.temp }}/digests/${digest#sha256:}"
     - name: Upload digest
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: digests-${{ inputs.suffix }}-${{ matrix.tag }}
         path: ${{ runner.temp }}/digests/*