Bump kotlin_version from 1.9.0 to 2.1.0

Bumps `kotlin_version` from 1.9.0 to 2.1.0.

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 1.9.0 to 2.1.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.1.0/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.0...v2.1.0)

Updates `org.jetbrains.kotlin:kotlin-stdlib` from 1.9.0 to 2.1.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.1.0/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.0...v2.1.0)

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

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

build.gradle

@@ -1,6 +1,6 @@
 buildscript {
     ext {
-        kotlin_version = '1.9.0'
+        kotlin_version = '2.1.0'
 	// wait for upstream https://github.com/mockito/mockito/issues/2007
         mockito_version = '3.4.6'
         threetenbp_version = '1.6.8'