build(deps): bump kotlin from 1.9.20 to 1.9.21

Bumps `kotlin` from 1.9.20 to 1.9.21.

Updates `org.jetbrains.kotlin.android` 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](https://github.com/JetBrains/kotlin/commits)

Updates `org.jetbrains.kotlin.plugin.serialization` 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](https://github.com/JetBrains/kotlin/commits)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin.android
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.plugin.serialization
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 agp = "8.3.0-alpha15"
-kotlin = "1.9.20"
+kotlin = "1.9.21"
 ksp = "1.9.21-1.0.15"
 core-ktx = "1.12.0"
 core-splashscreen = "1.0.1"