Document gcloudadmin and gcloud SDK hardening

In https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-workspace-management/-/issues/19 and documented operationally by @rk725 in https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-workspace-management/-/merge_requests/12, gcloudadmin account session lengths were minimised and access to some Google apps was removed.

Update the guidebook to include this information, probably in the explainer for why we have gcloudadmin accounts in the first place.