Cookies serializer improvements

- [x] Since the option actually changes how encrypted cookies are serialized, we should name it properly and include the signed cookie jar as well.
- [x] Better tests. Right now the test coverage is pretty sparse. Need to better test the existing cookie features and make sure they work with both of the built-in serializers.
- [x] Fix flash. (#13910, #13944)
- [ ] Find out what else is broken by swapping the default serializers, then fix those.
- [x] Add `HybridSerializer` (need a better name): subclass `JsonSerializer` to transparently migrate existing marshalled cookies into JSON cookies (we can detect the marshal signiture "\x04\x08" and branch accordingly).
- [x] Update docs, guides, changelogs to reflect this. Also mention the restrictions on what can be reliably stored in the serialized jars.
- [x] Upgrade guides on migrating existing cookies.