Kotlin: Fix custom_plugin test with Kotlin 2.0.0

Author: igfoo

java/ql/integration-tests/linux-only/kotlin/custom_plugin/test.py

@@ -3,4 +3,4 @@
 
 subprocess.call("./build_plugin", shell=True)
 run_codeql_database_create(
-    ["kotlinc -J-Xmx2G -Xplugin=plugin.jar a.kt b.kt c.kt d.kt e.kt"], lang="java")
+    ["kotlinc -J-Xmx2G -language-version 1.9 -Xplugin=plugin.jar a.kt b.kt c.kt d.kt e.kt"], lang="java")