Showing a boolean flag in the package JSON API for deprecation

[Kleidukos]

Currently, when viewing the JSON representation of a deprecated release like https://hackage.haskell.org/package/accelerate-cuda-0.15.1.1.json, there is no marker that the release has been deprecated, despite the fact that the package JSON will show the deprecation (https://hackage.haskell.org/package/accelerate-cuda.json)

There should be a boolean flag to show that the release is deprecated.

@gbaz I'd like to implement this. Is there anything I should be aware of while doing this?