Update kotlin to v2.0.21

Author: renovate[bot]

pom.xml

@@ -14,7 +14,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <java.version>11</java.version>
-        <kotlin.version>2.0.20</kotlin.version>
+        <kotlin.version>2.0.21</kotlin.version>
         <kotlin-coroutines.version>1.9.0</kotlin-coroutines.version>
         <jackson.version>2.18.0</jackson.version>
         <graphql-java.version>22.3</graphql-java.version>