Standardize on a single version of terraform

Created by: sqs

The https://github.com/sourcegraph/infrastructure repo uses 2 different terraform versions for cloud (0.14.11) vs dns (0.13.7). I also see we're thinking of upgrading managed to 1.x (https://github.com/sourcegraph/sourcegraph/issues/22905). It would be nice to have a single version so we don't need to download multiple. (It wasn't super hard, but would be nice. :)