ci: bump docker/build-push-action from 5 to 6

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 5 to 6.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@v5...v6)

---
updated-dependencies:
- dependency-name: docker/build-push-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/benchmarking-image.yml

@@ -70,7 +70,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: "Rebuild ghcri.io/oasisprotocol/oasis-sdk-benchmark:${{ steps.determine_tag.outputs.tag }}"
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: tests/benchmark/Dockerfile

.github/workflows/runtime-builder-image.yml

@@ -65,7 +65,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: "Rebuild oasisprotocol/runtime-builder:${{ steps.determine-tag.outputs.tag }}"
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: docker/runtime-builder
           file: docker/runtime-builder/Dockerfile