Upgrade Kotlin from 2.2.20 to 2.2.21 for Xcode 26 compatibility

Kotlin 2.2.20 has known compatibility issues when building on Xcode 26.x.
Official support for Xcode 26 was introduced in Kotlin 2.2.21.

This change upgrades Kotlin to 2.2.21 to ensure compatibility with
our macOS 26.x and Xcode 26.x build environment.

Author: ghoshArnab

gradle/libs.versions.toml

@@ -13,7 +13,7 @@ androidx-test-junit = "1.3.0"
 compose-hot-reload = "1.0.0"
 compose-multiplatform = "1.9.0" # warning: update bumps kotlinx-datetime, which breaks iOS
 compose-navigation = "2.9.1"
-kotlin = "2.2.20"
+kotlin = "2.2.21"
 kotlinx-coroutines = "1.10.2"
 kotlinx-datetime = "0.6.2" # warning: update breaks iOS compilation
 kotlinx-serialization-json = "1.9.0"