Update kotlin monorepo to v2.1.21 (#2318)

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

gradle/libs.versions.toml

@@ -19,7 +19,7 @@ arrow = "2.1.1"
 coil = "3.2.0"
 compose-bom = "2025.05.00"
 compose-destinations = "2.2.0"
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 kotlinx-coroutines = "1.10.2"
 kotlinx-datetime = "0.6.2"
 kotlinx-serialization = "1.8.1"