[Bazel] Update to 4.2.1

[keith]

Summary

In order to build on Apple Silicon machines this is a requirement. This
causes a warning because of
https://github.com/bazelbuild/bazel-toolchains/issues/970 but is
probably still worth it

[GMNGeoffrey]

I'll need to install 4.2.1 on the CI machines. I also don't like the warnings and would prefer that we wait until that's fixed upstream. In the meantime, you can actually run this fine with bazel-4.2.1, you just have to invoke the binary directly instead of the bazel shell script that comes with the bazel install by default and picks a bazel version based on the .bazelversion file

[GMNGeoffrey]

I don't think you need to abandon :-) I'm happy to do the bump: I just really hate warning spam and would prefer we fix that first

[keith]

We can pick back up if that warning is ever fixed. For now using bazelisk + USE_BAZEL_VERSION is fine for me, just wanted to fix for others as well.

utils/bazel/.bazelversion

-4.0.0
+4.2.1