Update kotlin (#23)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

gradle/libs.versions.toml

@@ -1,14 +1,14 @@
 [versions]
 
-kotlin = "2.1.0"
+kotlin = "2.1.20"
 jdkTarget = "8"
 jvmToolchain = "21"
 kotlinTarget = "1.9.0"
 ktor2 = "2.3.13"
 ktor3 = "3.0.1"
-kotlinx-serialization = "1.7.3"
-kotlinx-datetime = "0.6.1"
-poko = "0.18.0"
+kotlinx-serialization = "1.8.1"
+kotlinx-datetime = "0.6.2"
+poko = "0.18.6"
 kermit = "2.0.5"
 spotless = "7.0.3"
 vanniktech-maven = "0.31.0"