In future we will support version constraints on providers, so we're
reserving this attribute name that is currently not used by any builtin
providers.
For now using this will produce an error, since the rest of Terraform
(outside of the config parser) doesn't currently have this notion and we
don't want people to start trying to use it until its behavior is fully
defined and implemented.
It may be used by third-party providers, so this is a breaking change
worth warning about in CHANGELOG but one whose impact should be small.
Any third-party providers using this name should migrate to using a new
attribute name instead moving forward.
