Kotlin: make 2.4.20-RC the test default

Select the supported release candidate by default and update the generated too-new diagnostic expectation for the new exclusive version bound.

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():

java/ql/integration-tests/kotlin/all-platforms/diagnostics/kotlin-version-too-new/diagnostics.expected

@@ -1,5 +1,5 @@
 {
-  "markdownMessage": "The Kotlin version installed (`999.999.999`) is too recent for this version of CodeQL. Install a version lower than 2.4.20.",
+  "markdownMessage": "The Kotlin version installed (`999.999.999`) is too recent for this version of CodeQL. Install a version lower than 2.4.21.",
   "severity": "error",
   "source": {
     "extractorName": "java",