Kotlin: make 2.4.20-RC the test default

Use Kotlin 2.4.20-RC by default in the development wrapper so Kotlin 2 tests exercise the newly supported prerelease.

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():