Kotlin-Client: Fix kotlinx_serialization

Created by: saschpe

And update to Kotlin 1.5.0 and kotlinx.serialization 1.2.1. Fix nested enum annotation '@Serializable' instead of '@KSerializable' when 'kotlinx_serialization' is used. Fix missing JsonMediaType in ApiClient.kt (#9242 (closed)). Add 'kotlinx_serialization' serialization library to documentation.

Resolves #9242 (closed)

@jimschubert @rsinukov @wing328 @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

Tested with serializationLibrary set to gson, kotlinx_serialization and default (moshi) as well as library set to multiplatform and default (jvm-okhttp4).

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