Upgrade bazel to 8.0.0.

Previously, we were using 8.0.0rc1.
In particular, this upgrade means we need to explicitly
import more rules, as they've been moved out of the core bazel repo.

rust/codegen/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@bazel_skylib//rules:native_binary.bzl", "native_binary")
+load("@rules_shell//shell:sh_binary.bzl", "sh_binary")
 
 _args = [
     "//rust/ast-generator",