Bump org.jetbrains.kotlin.multiplatform from 2.1.20 to 2.1.21

Bumps [org.jetbrains.kotlin.multiplatform](https://github.com/JetBrains/kotlin) from 2.1.20 to 2.1.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.1.20...v2.1.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin.multiplatform
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 android-gradle-plugin = "8.9.2"
 dokka = "2.0.0"
 spotless = "7.0.3"