Upgrade to Bazel 8

Bazel 8.0 LTS just got released: https://github.com/bazelbuild/bazel/releases/tag/8.0.0
However, we're still running on 5.4.0: https://github.com/p4lang/p4-constraints/blob/140ac6796bb01134246de8e1a9297dd2e435d6a1/.bazelversion

This issue tracks upgrading to Bazel 8 to fight bit rot.