Pin Kotlin to 2.2.20 (CodeQL compatibility)

Kotlin 2.4.0 builds fine but breaks CodeQL analysis
(KotlinVersionTooRecentError; CodeQL supports < 2.3.30). Pin to the
newest CodeQL-supported release so security scanning stays green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jochen-testingbot

gradle/libs.versions.toml

@@ -1,5 +1,7 @@
 [versions]
-kotlin = "2.4.0"
+# Kotlin is pinned below 2.3.30: the CodeQL Kotlin extractor does not yet support
+# newer releases (fails with KotlinVersionTooRecentError). Bump once CodeQL catches up.
+kotlin = "2.2.20"
 gson = "2.14.0"
 junit = "6.1.1"
 pluginPublish = "2.1.1"