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-gradle-plugin` 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-allopen` 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-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-reflect` 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)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-allopen
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib-jdk8
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-reflect
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle

@@ -1,6 +1,6 @@
 buildscript {
     ext {
-        kotlin_version = "1.9.20"
+        kotlin_version = "1.9.21"
         rabbitmq_java_client_version = "5.20.0"
         junit_jupiter_version = "5.10.1"
         testcontainers_version = "1.19.2"