Deprecated method for pinning provider versions

[bbchase]

The using-terraform-modules.md doc suggests pinning the provider version in the provider block, but this method is deprecated. Provider version constraints should now be in the required_providers block. (See line 42)

The service catalogue has already been updated (e.g. vpc-app, which is used as the example for this doc), but the docs themselves appear to be out of date.

terraform warning message:

Warning: Version constraints inside provider configuration blocks are deprecated

  on provider.tf line 5, in provider "aws":
   5:   version = "~> 3.0"

Terraform 0.13 and earlier allowed provider version constraints inside the
provider configuration block, but that is now deprecated and will be removed
in a future version of Terraform. To silence this warning, move the provider
version constraint into the required_providers block.

[eak12913]

Thanks @bbchase for the submission and my apologies that this took so long for us to get to.