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-stdlib` 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-stdlib
  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"
         KETHEREUM_VERSION = "0.86.0"
     }