Fix compilation warnings after Kotlin 2.4.20 and KSP 2.3.12 upgrade

- Update .idea/kotlinc.xml to Kotlin 2.4.20.
- Remove `-Xannotation-default-target=first-only`: the new annotation
  default target rule is stable in Kotlin 2.4, and disabling it now
  produces a CLI_ARG_DISABLES_STABLE_FEATURE warning.
- Replace deprecated `KSNode.validate()` with
  `validate(enableNewFeatures = false)` to keep the same behavior.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

codegen/src/main/kotlin/io/element/android/codegen/ContributesNodeProcessor.kt

@@ -53,7 +53,7 @@ class ContributesNodeProcessor(
         val annotatedSymbols = resolver.getSymbolsWithAnnotation(ContributesNode::class.qualifiedName!!)
             .filterIsInstance<KSClassDeclaration>()
 
-        val (validSymbols, invalidSymbols) = annotatedSymbols.partition { it.validate() }
+        val (validSymbols, invalidSymbols) = annotatedSymbols.partition { it.validate(enableNewFeatures = false) }
 
         if (validSymbols.isEmpty()) return invalidSymbols