fix: samples/server/petstore/kotlin-springboot-modelMutable/pom.xml t…

…o reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-ORGJETBRAINSKOTLIN-2393744

samples/server/petstore/kotlin-springboot-modelMutable/pom.xml

@@ -6,7 +6,7 @@
     <name>openapi-spring</name>
     <version>1.0.0</version>
     <properties>
-        <kotlin.version>1.3.30</kotlin.version>
+        <kotlin.version>2.0.21</kotlin.version>
 	    <kotlinx-coroutines.version>1.2.0</kotlinx-coroutines.version>
         <javax-annotation-version>1.3.2</javax-annotation-version>
     </properties>