[CI] Bump docker/build-push-action from 6 to 7 (#7541)

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 6 to 7.

- vLLM version: v0.18.0
- vLLM main:
8b6325758c

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

@@ -77,7 +77,7 @@ jobs:
         use: true
 
     - name: Build and push
-      uses: docker/build-push-action@v6
+      uses: docker/build-push-action@v7
       id: build
       with:
         platforms: ${{ matrix.arch }}