Remove support for open polymorphic serialization in
kotlinx.serialization web converters and codecs in order
to prevent serialization handling suitable for Jackson
or other general purpose Java JSON libraries.
This will probably need further refinements for collections
for example, and could ultimately be fixed when
kotlinx.serialization will provide a dedicated function to
evaluate upfront if a type can be serialized or not.
Closes gh-26298
2d53570f4c