Kotlin: make 2.4.20-RC the test default

Use the published RC by default in the development wrapper so the Kotlin 2
suite exercises the compiler compatibility changes from KT-85816 and
KT-86046.

Use Kotlin 2.4.20 GA in the too-new diagnostic fixture to verify that support
remains limited to the RC.

Kotlin changelog:
https://github.com/JetBrains/kotlin/blob/v2.4.20-RC/ChangeLog.md#2420-beta2

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: andersfugmann

java/kotlin-extractor/dev/wrapper.py

@@ -27,7 +27,7 @@
 import io
 import os
 
-DEFAULT_VERSION = "2.4.10"
+DEFAULT_VERSION = "2.4.20-RC"
 
 
 def options():