Instead of using providers.tf as the default output file for the
upgrader, we now default to versions.tf. This means that if the
configuration has no `required_providers` blocks at all, or has
multiple, the provider version requirements will be stored in the
versions.tf file.
We now also update the versions.tf file to set a `required_version`
attribute in the first `terraform` block, with value ">= 0.13". This
is similar to the behaviour of the 0.12upgrade command, and signals that
the configuration should not be used with older versions of Terraform.
a740b739e0