Update Kotlin version to 1.9.20

Author: FenstonSingel

README.md

@@ -4,7 +4,7 @@
 [![JetBrains official project](https://jb.gg/badges/official.svg)](https://confluence.jetbrains.com/display/ALL/JetBrains+on+GitHub) 
 [![GitHub license](https://img.shields.io/badge/license-Apache%20License%202.0-blue.svg?style=flat)](http://www.apache.org/licenses/LICENSE-2.0)
 [![Maven Central](https://img.shields.io/maven-central/v/org.jetbrains.kotlinx/kotlinx-datetime.svg?label=Maven%20Central)](https://search.maven.org/search?q=g:%22org.jetbrains.kotlinx%22%20AND%20a:%22kotlinx-datetime%22)
-[![Kotlin](https://img.shields.io/badge/kotlin-1.8.21-blue.svg?logo=kotlin)](http://kotlinlang.org)
+[![Kotlin](https://img.shields.io/badge/kotlin-1.9.20-blue.svg?logo=kotlin)](http://kotlinlang.org)
 [![KDoc link](https://img.shields.io/badge/API_reference-KDoc-blue)](https://kotlinlang.org/api/kotlinx-datetime/)
 [![Slack channel](https://img.shields.io/badge/chat-slack-blue.svg?logo=slack)](https://kotlinlang.slack.com/messages/kotlinx-datetime/)
 [![TeamCity build](https://img.shields.io/teamcity/build/s/KotlinTools_KotlinxDatetime_Build_All.svg?server=http%3A%2F%2Fteamcity.jetbrains.com)](https://teamcity.jetbrains.com/viewType.html?buildTypeId=KotlinTools_KotlinxDatetime_Build_All&guest=1)

gradle.properties

@@ -5,7 +5,7 @@ group=org.jetbrains.kotlinx
 version=0.4.1
 versionSuffix=SNAPSHOT
 
-defaultKotlinVersion=1.8.21
+defaultKotlinVersion=1.9.20
 dokkaVersion=1.8.20
 serializationVersion=1.5.1