fix: samples/server/petstore/kotlin-springboot-modelMutable/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-ORGSPRINGFRAMEWORK-8384234
- https://snyk.io/vuln/SNYK-JAVA-ORGJETBRAINSKOTLIN-2393744

Author: snyk-bot

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

@@ -11,7 +11,7 @@
         <javax-annotation.version>1.3.2</javax-annotation.version>
         <kotlin-test-junit5.version>1.6.21</kotlin-test-junit5.version>
 
-        <kotlin.version>1.6.21</kotlin.version>
+        <kotlin.version>2.1.0</kotlin.version>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     </properties>
     <parent>