Bump PyArrow Version to 6
_The documentation is not available anymore as the PR was closed or merged._ Updated meta.yaml as well. Thanks. I'm OK with bumping PyArrow to version 6 to match the version in Colab, but maybe a better solution would be to stop using extension types in our codebase to avoid similar issues. > but maybe a better solution would be to stop using extension types in our codebase to avoid similar issues.

I agree, not much attention has been payed to extension arrays in the latest developments of Arrow anyway.

Let's not use them more that what we do right now, and try to remove them at one point
Fixes #4152 

This PR updates the PyArrow version to 6 in setup.py, CI job files .circleci/config.yaml and .github/workflows/benchmarks.yaml files.
This will fix ArrayND error which exists in pyarrow 5.