Bump kotlin.version from 1.9.20 to 1.9.21

Bumps `kotlin.version` from 1.9.20 to 1.9.21.

Updates `org.jetbrains.kotlin:kotlin-stdlib-jdk8` from 1.9.20 to 1.9.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.20...v1.9.21)

Updates `org.jetbrains.kotlin:kotlin-test` from 1.9.20 to 1.9.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.20...v1.9.21)

Updates `org.jetbrains.kotlin:kotlin-test-junit` from 1.9.20 to 1.9.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.20...v1.9.21)

Updates `org.jetbrains.kotlin:kotlin-maven-plugin` from 1.9.20 to 1.9.21

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib-jdk8
  dependency-type: direct:development
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-test
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-test-junit
  dependency-type: direct:development
  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] <support@github.com>

pom.xml

@@ -102,7 +102,7 @@
     <java.version>17</java.version>
     <junit-jupiter.version>5.7.2</junit-jupiter.version>
     <junit.version>4.13.2</junit.version>
-    <kotlin.version>1.9.20</kotlin.version>
+    <kotlin.version>1.9.21</kotlin.version>
     <license-maven-plugin-git.version>4.3</license-maven-plugin-git.version>
     <maven-checkstyle-plugin.version>3.3.1</maven-checkstyle-plugin.version>
     <maven-clean-plugin.version>3.3.2</maven-clean-plugin.version>