fix: remove unspecified from consistency parameter documentation

[jimmyjames]

Description

Sets the UNSPECIFIED consistency preference to INTERNAL, which prevents it from being exported as a valid enum value in the swagger.

SDKs can then be generated without requiring any odd manipulation to handle namespace collisions on UNSPECIFIED.

References

https://grpc-ecosystem.github.io/grpc-gateway/docs/mapping/customizing_openapi_output/#hiding-fields-methods-services-and-enum-values

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected