Update kotlin monorepo to v2.1.20

Author: renovate[bot]

gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 autoService = "1.1.1"
 jdk = "21"
 jvmTarget = "1.8"
-kotlin = "2.0.0"
+kotlin = "2.1.20"
 # No 0.5.0 full release yet due to KSP's 1.0.21 release being busted for CLI/programmatic use
 kotlinCompileTesting = "0.5.0-alpha07"
 kotlinpoet = "2.1.0"