Update gsutil to a newer version to resolve some compatibility issues
with newer Python versions.

And update the README to reflect the new changes.

Bug: 40283283
Change-Id: I1e02c6868e932da0af45a631fc34539c3543868e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/7058090


Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Ho Cheung <hocheung@chromium.org>
Reviewed-by: Brian Ryner <bryner@google.com>