Update kotlin to v1.9.21

Author: renovate[bot]

gradle/libs.versions.toml

@@ -8,7 +8,7 @@ rewrite = "7.40.8"
 rewrite-testing = "1.37.0"
 openrewrite = "6.0.0"
 assertj = "3.24.2"
-kotlin = "1.8.21"
+kotlin = "1.9.21"
 nexus = "1.3.0"
 checkstyle = "10.12.0"