Bump kotlin_version from 1.8.21 to 1.9.0 (#149)

Bumps `kotlin_version` from 1.8.21 to 1.9.0.

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 1.8.21 to 1.9.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.8.21...v1.9.0)

Updates `org.jetbrains.kotlin:kotlin-stdlib` from 1.8.21 to 1.9.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.8.21...v1.9.0)

Updates `org.jetbrains.kotlin:kotlin-test-junit` from 1.8.21 to 1.9.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.8.21...v1.9.0)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

build.gradle

@@ -1,7 +1,7 @@
 version '1.0-SNAPSHOT'
 
 buildscript {
-    ext.kotlin_version = '1.8.21'
+    ext.kotlin_version = '1.9.0'
     ext.dokka_version = '1.8.20'
     ext.bnd_version = '6.4.0'
     ext.license_version = '0.16.1'