Use API to determine available Terraform versions, not HTML scraping

[tevert]

See comment: hashicorp/terraform#9803 (comment)

In the spirit of Seth's comment, it might also make sense to change the Version field to a dropdown of major/minor versions. But then I'd need to be constantly adding new dropdown items....

[giuliov]

In my Extension, I use the GitHub API to determine the latest Release available for Hugo. Sadly, in GitHub there are no binaries for terraform, but the version numbers should match.