Update kotlin to v2.2.21 (#207)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

gradle/libs.versions.toml

@@ -1,7 +1,7 @@
 [versions]
 
 # --- Kotlin & Core ---
-kotlin = "2.2.20"            # https://github.com/JetBrains/kotlin
+kotlin = "2.2.21"            # https://github.com/JetBrains/kotlin
 coroutines = "1.10.2"        # https://github.com/Kotlin/kotlinx.coroutines
 serialization = "1.9.0"      # https://github.com/Kotlin/kotlinx.serialization
 kapt = "2.1.0"               # https://kotlinlang.org/docs/kapt.html