Update kotlin monorepo to v2.1.21

Author: renovate[bot]

gradle/libs.versions.toml

@@ -3,7 +3,7 @@ agp = "8.10.0"               # keep in sync with android-tools
 android-tools = "31.10.0"    # = 23.0.0 + agp
 compilerTesting = "0.7.1"
 compose = "1.5.14"
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 kotlinPoet = "1.13.1"
 ksp = "2.1.20-1.0.32"
 ktlint = "0.46.1"