Mention Kotlin 1.6.21 in the README

Author: dkhalanskyjb

README.md

@@ -92,7 +92,7 @@ And make sure that you use the latest Kotlin version:
 
 ```xml
 <properties>
-    <kotlin.version>1.6.20</kotlin.version>
+    <kotlin.version>1.6.21</kotlin.version>
 </properties>
 ```
 
@@ -111,10 +111,10 @@ And make sure that you use the latest Kotlin version:
 ```kotlin
 plugins {
     // For build.gradle.kts (Kotlin DSL)
-    kotlin("jvm") version "1.6.20"
+    kotlin("jvm") version "1.6.21"
 
     // For build.gradle (Groovy DSL)
-    id "org.jetbrains.kotlin.jvm" version "1.6.20"
+    id "org.jetbrains.kotlin.jvm" version "1.6.21"
 }
 ```