`ruby-graphql` supports

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/27464 added support
for a custom `deprecated` keyword for `field` to allow us to specify the
milestone that a field was deprecated in a structured way.

Enum values are also able to be deprecated. This change adds support for
enum values.

Also see: https://gitlab.com/gitlab-org/gitlab/-/issues/199952