Use Kotlin 2.3.20 for IntelliJ 2026.1+.

Author: rhdunn

buildSrc/src/main/kotlin/Version.kt

@@ -24,6 +24,7 @@ object Version {
      */
     @Suppress("FunctionName")
     fun Kotlin(intellij: IntelliJVersion): String = when {
+        intellij.buildVersion >= 261 -> "2.3.20"
         intellij.buildVersion >= 251 -> "2.1.10"
         intellij.buildVersion >= 242 -> "1.9.22"
         intellij.buildVersion >= 223 -> "1.8.22"