Upgrade to terraform v0.12

This will remove depreciation warning received when using this module in 0.12.

New protected branch "v0.11" has been created

To be used in projects still using terraform 0.11, via:

  source = "git::https://gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gcp-project.git?ref=v0.11"

Closes #3 (closed)