Bump version in README (#1949)

Author: sandwwraith

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)
 [![TeamCity build](https://img.shields.io/teamcity/http/teamcity.jetbrains.com/s/KotlinTools_KotlinxSerialization_Ko.svg)](https://teamcity.jetbrains.com/viewType.html?buildTypeId=KotlinTools_KotlinxSerialization_Ko&guest=1)
-[![Kotlin](https://img.shields.io/badge/kotlin-1.6.10-blue.svg?logo=kotlin)](http://kotlinlang.org)
+[![Kotlin](https://img.shields.io/badge/kotlin-1.6.21-blue.svg?logo=kotlin)](http://kotlinlang.org)
 [![Maven Central](https://img.shields.io/maven-central/v/org.jetbrains.kotlinx/kotlinx-serialization-core/1.3.3)](https://search.maven.org/artifact/org.jetbrains.kotlinx/kotlinx-serialization-core/1.3.3/pom)
 [![KDoc link](https://img.shields.io/badge/API_reference-KDoc-blue)](https://kotlin.github.io/kotlinx.serialization/)
 [![Slack channel](https://img.shields.io/badge/chat-slack-blue.svg?logo=slack)](https://kotlinlang.slack.com/messages/serialization/)
@@ -89,17 +89,17 @@ Kotlin DSL:
 
 ```kotlin
 plugins {
-    kotlin("jvm") version "1.6.10" // or kotlin("multiplatform") or any other kotlin plugin
-    kotlin("plugin.serialization") version "1.6.10"
+    kotlin("jvm") version "1.6.21" // or kotlin("multiplatform") or any other kotlin plugin
+    kotlin("plugin.serialization") version "1.6.21"
 }
 ```       
 
 Groovy DSL:
 
 ```gradle
 plugins {
-    id 'org.jetbrains.kotlin.multiplatform' version '1.6.10'
-    id 'org.jetbrains.kotlin.plugin.serialization' version '1.6.10'
+    id 'org.jetbrains.kotlin.multiplatform' version '1.6.21'
+    id 'org.jetbrains.kotlin.plugin.serialization' version '1.6.21'
 }
 ```
 
@@ -116,7 +116,7 @@ buildscript {
     repositories { mavenCentral() }
 
     dependencies {
-        val kotlinVersion = "1.6.10"
+        val kotlinVersion = "1.6.21"
         classpath(kotlin("gradle-plugin", version = kotlinVersion))
         classpath(kotlin("serialization", version = kotlinVersion))
     }
@@ -127,7 +127,7 @@ Groovy DSL:
 
 ```gradle
 buildscript {
-    ext.kotlin_version = '1.6.10'
+    ext.kotlin_version = '1.6.21'
     repositories { mavenCentral() }
 
     dependencies {
@@ -286,7 +286,7 @@ Ensure the proper version of Kotlin and serialization version:
 
 ```xml
 <properties>
-    <kotlin.version>1.6.10</kotlin.version>
+    <kotlin.version>1.6.21</kotlin.version>
     <serialization.version>1.3.3</serialization.version>
 </properties>
 ```