Stabilize most of SerialDescriptor and SerialKind API

This API has been around for a long time and has proven itself useful. The main reason @ExperimentalSerializationApi was on SerialDescriptor's properties is that we wanted to discourage people from subclassing it. With the introduction of @SubclassOptInRequired (#2366), we can do this without the need of marking everything with experimental.

Serial kinds fall into the same category with only exception in PolymorphicKind. There are plenty requests for functionality like creating a custom sealed-like descriptor (#2697, #2721, #1865) which may require additional kinds in the future.