[Kotlin Multiplatform] Update and fixes for Kotlin 1.5.10 and Kotlinx.serialization 1.2.1

[837]

• Update Kotlin Multiplatform with Kotlin 1.5.10, Ktor 1.6.0 and Kotlinx.serialization 1.2.1
• remove @serializable from interfaces
• Use CharArray.concatToString() instead of 'String(CharArray): String' as it is an error now
• Fix, RequestConfig needs a type to infer type variable T

Fixes from @DevSrSouzaern #9232
Seemed not to be updated anymore.

• Fix private keyword not being handle as a reservedWords
• Fix enum serialization generation
• Migrate gradle build to Kotlin DSL

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
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