Update kotlin to v1.8.21

Author: renovate[bot]

pom.xml

@@ -14,7 +14,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <java.version>1.8</java.version>
-        <kotlin.version>1.8.10</kotlin.version>
+        <kotlin.version>1.8.21</kotlin.version>
         <kotlin-coroutines.version>1.6.4</kotlin-coroutines.version>
         <jackson.version>2.14.2</jackson.version>
         <graphql-java.version>20.0</graphql-java.version>