This has some extra affects that required a little more changes than the other migrations did:

First, `MediaObject` was an open base class shared via inheritance by `HistoryMediaObject`/`RatingMediaObject`/`StatusMediaObject`. kotlinx doesn't walk inherited properties the way Jackson does:
* A super class requires either `SerialName` or `@Transient`
* A sub class can not have the same `@SerialName` as the super class due to JSON conflict
* That would mean, the super class would need `@Transient` which works for abstract but not really open classes, because an open class may be used standalone also, and `@Transient` prevents that particular field from being what triggers serialization, which means without a sub class it wouldn't work at all.

Therefore, to resolve this issue, I switched to flat, independent data classes (composition instead of inheritance), and split `StatusRequest` into `StatusRequest`/`HistoryRequest` so each request body is typed to its concrete element type.

Also, this new way makes the code a little more explicitly and easier to understand IMO anyway.

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Secondly, `SimklCacheWrapper<T>` was generic, but kotlinx needs a concrete serializer per type, which generics erase at runtime. Therefore for me to resolve this, I replaced it with two concrete wrapper classes (`MediaObjectCacheEntry` and `EpisodesCacheEntry`) since those were the only two things ever cached, plus a small `CacheFreshness` class for the generic "check any entry's expiry".

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I did also fully test this but I suppose there is a possibility I could have missed something.