Bump actions/upload-artifact from 6 to 7 (#6906)

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 6 to 7.

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

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

.github/workflows/_schedule_image_build.yaml

@@ -98,7 +98,7 @@ jobs:
         touch "${{ runner.temp }}/digests/${digest#sha256:}"
 
     - name: Upload digest
-      uses: actions/upload-artifact@v6
+      uses: actions/upload-artifact@v7
       with:
         name: digests-${{ inputs.suffix }}-${{ matrix.tag }}
         path: ${{ runner.temp }}/digests/*