Bump kotlin.version from 2.0.20 to 2.0.21

Bumps `kotlin.version` from 2.0.20 to 2.0.21.

Updates `org.jetbrains.kotlin:kotlin-bom` from 2.0.20 to 2.0.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.0.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.20...v2.0.21)

Updates `org.jetbrains.kotlin:kotlin-maven-plugin` from 2.0.20 to 2.0.21

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-bom
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-maven-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

pom.xml

@@ -7,7 +7,7 @@
     <version>1.0.0</version>
     <packaging>jar</packaging>
     <properties>
-        <kotlin.version>2.0.20</kotlin.version>
+        <kotlin.version>2.0.21</kotlin.version>
         <vertx.version>4.5.10</vertx.version>
         <jackson-bom.version>2.18.0</jackson-bom.version>
         <java.version>21</java.version>