Merge pull request #437 from usefulness/renovate/kotlin

Update kotlin to v2.1.21

Author: renovate[bot]

gradle/libs.versions.toml

@@ -17,7 +17,7 @@ byteunits = "0.9.1"
 asm = "9.8"
 javaDiffUtils = "4.15"
 picnic = "0.7.0"
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 ktlint-gradle-plugin = "0.10.0"
 r8 = "8.7.18"
 assertJ = "3.27.3"