Update Google Cloud KMS to latest version

[rtibbles]

Version 2.0.0 introduced breaking changes to the API, so to keep dependencies up to date, we would need to update our usage of this library.

This would involve updating usage of the library in this file: https://github.com/learningequality/studio/blob/unstable/contentcuration/contentcuration/utils/secretmanagement.py

And ensuring the invocation is correct here: https://github.com/learningequality/studio/blob/unstable/k8s/encrypt-env-var.sh

Following the migration guide here: https://github.com/googleapis/python-kms/blob/release-v2.0.0/UPGRADING.md