[dependency] Bump kotlin.version from 1.8.10 to 1.9.21

Bumps `kotlin.version` from 1.8.10 to 1.9.21.

Updates `org.jetbrains.kotlin:kotlin-stdlib-jdk8` from 1.8.10 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@v1.8.10...v1.9.21)

Updates `org.jetbrains.kotlin:kotlin-test` from 1.8.10 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@v1.8.10...v1.9.21)

Updates `org.jetbrains.kotlin:kotlin-maven-plugin` from 1.8.10 to 1.9.21

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib-jdk8
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.jetbrains.kotlin:kotlin-test
  dependency-type: direct:development
  update-type: version-update:semver-minor
- dependency-name: org.jetbrains.kotlin:kotlin-maven-plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

pom.xml

@@ -39,7 +39,7 @@
     <!-- plugins -->
     <maven-compiler.version>3.8.1</maven-compiler.version>
     <maven-surefire.version>2.22.2</maven-surefire.version>
-    <kotlin.version>1.8.10</kotlin.version>
+    <kotlin.version>1.9.21</kotlin.version>
     <mockk.version>1.13.4</mockk.version>
     <kotest.version>5.5.5</kotest.version>
   </properties>