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/build.yml

@@ -60,7 +60,7 @@ jobs:
         id: meta
         with:
           images: ghcr.io/${{ github.repository_owner }}/ruby.wasm/builder/${{ matrix.entry.target }}
-      - uses: docker/build-push-action@v5
+      - uses: docker/build-push-action@v6
         with:
           context: builders/${{ matrix.entry.target }}
           push: ${{ github.ref == 'refs/heads/main' || github.ref == 'refs/tags/*' }}
@@ -113,7 +113,7 @@ jobs:
           path: ./rubies
           key: ${{ matrix.entry.rubies_cache_key }}
       - uses: docker/setup-buildx-action@v3
-      - uses: docker/build-push-action@v5
+      - uses: docker/build-push-action@v6
         id: builder-image
         with:
           context: builders/${{ matrix.entry.builder }}