build(deps): bump kotlinVersion from 1.9.20 to 1.9.21

Bumps `kotlinVersion` from 1.9.20 to 1.9.21.

Updates `plugin.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.jvm` 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.plugin.spring` 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: plugin.allopen
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.jvm
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.plugin.spring
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle.kts

@@ -26,7 +26,7 @@ java {
 plugins {
  application
  java
- val kotlinVersion = "1.9.20"
+ val kotlinVersion = "1.9.21"
  kotlin("plugin.allopen") version kotlinVersion
  id("org.jmailen.kotlinter") version "3.10.0"
  id("com.github.ben-manes.versions") version "0.49.0"