update bazel to 8.6.0 - #306

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4.2.2, bazel-contrib/setup-bazel@0.18.0, docker/setup-buildx-action@v3.8.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/