[llvm] [bazel] update .bazelversion to 8.0.0 (PR #119425)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Hmm...even with this change, I'm observing:

```
$ cd llvm-project/utils/bazel
$ bazel query @llvm-project//libc/... | xargs bazel test --config=generic_clang --test_output=errors --test_tag_filters=-nobuildkite --build_tag_filters=-nobuildkite -- at llvm-project//libc:mpfr=system
ERROR: Error computing the main repository mapping: error loading package 'external': Both --enable_bzlmod and --enable_workspace are disabled, but one of them must be enabled to fetch external dependencies.
ERROR: Error computing the main repository mapping: error loading package 'external': Both --enable_bzlmod and --enable_workspace are disabled, but one of them must be enabled to fetch external dependencies.
```

https://github.com/llvm/llvm-project/pull/119425