Update kotlin monorepo to v2.1.21

Author: renovate[bot]

gradle/libs.versions.toml

@@ -27,7 +27,7 @@ androidx-test-rules = "1.6.1"
 
 junit-version = "4.13.2"
 
-kotlin = "2.0.21"
+kotlin = "2.1.21"
 agp-version = "7.4.2"
 flank-version = "23.10.1"