postage: replaced by pgmanage-10.0.2

postage is no longer maintained and has been replaced by the identical pgmanage. See:

https://github.com/workflowproducts/postage#postage-has-been-replaced-with-pgmanage

The following error is raised when a user enables the deprecated `services.postage.enable` option:

Failed assertions:
- services.postage is deprecated in favor of pgmanage. They have the same options so just substitute postage for pgmanage.