chore(deps): update kotlin monorepo to v2.1.21

Author: renovate[bot]

AndroidApp/gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 agp = "8.9.1"
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 com-google-devtools-ksp = "2.1.20-2.0.1" # Kotlinのバージョンに合わせる必要がある
 com-jaredsburrows-license = "0.9.8"
 androidx-core = "1.16.0"