deps: update kotlin monorepo to v2.1.21

Author: renovate[bot]

gradle/libs.versions.toml

@@ -8,7 +8,7 @@ gitSemverPlugin = "0.16.0"
 graalVmNativeImagePlugin = "0.10.6"
 ideaExtPlugin = "1.1.10"
 jakartaMigrationPlugin = "0.24.0"
-kotlinPlugin = "2.1.20"
+kotlinPlugin = "2.1.21"
 ksp = "2.1.20-2.0.1"
 mavenPublishPlugin = "0.31.0"
 reproducibleBuildsPlugin = "1.0"