It was changed in https://github.com/rails/rails/commit/ead947a3b2bc672b6064a6d0d33905d45299d22e to enable automatica write-back.

Previously, I was passing a `HybridSerializer` downstream to the `Message{Encryptor,Verifier}`, however, if we want to actually migrate the cookie value on *read* (i.e. if I access `cookies.signed[:some_key]` it will re-write it as json), then it has to be done on the cookie jar level, because it needs access to `[]=` on the cookie jar.

That is mainly to mirror the behaviour on the legacy cookie jar migration thing we did from 3.2 -> 4.0, and to more aggressively migrate legacy cookies.

If we don't care for that, it could be much simpler.