Update Kotlin/Jupyter to v0.12.0-421

Author: renovate[bot]

gradle/libs.versions.toml

@@ -2,9 +2,9 @@
 kotlin = "2.1.20"
 dokka = "2.0.0"
 openapi-generator = "7.12.0"
-jupyter-api = "0.12.0-398"
-jupyter-testkit = "0.12.0-398"
-jupyter-plugin = "0.12.0-398"
+jupyter-api = "0.12.0-421"
+jupyter-testkit = "0.12.0-421"
+jupyter-plugin = "0.12.0-421"
 okio = "3.11.0"
 moshi = "1.15.2"
 okhttp = "4.12.0"