chore(android): bump Kotlin 2.2 -> 2.3 + KSP 2.0 -> 2.3 for AGP 9 built-in Kotlin

The previous attempt opted out of AGP 9's built-in Kotlin (via
android.builtInKotlin=false + explicit kotlin-android plugin) because
the message from Gradle suggested it. But Kotlin 2.2.21's
kotlin-android plugin can't cast AGP 9's new ApplicationExtension to
the removed BaseExtension:

  class ApplicationExtensionImpl$AgpDecorated_Decorated
  cannot be cast to class com.android.build.gradle.BaseExtension

That suggestion is for projects with an older Kotlin toolchain. The
real fix:

  - Kotlin 2.3.21 (latest stable; first line where kotlin-android also
    supports AGP 9, but more importantly the built-in path works)
  - KSP 2.3.8 — KSP PR #2674 (merged Oct 2025) added AGP 9 built-in
    Kotlin support. KSP 1.x and pre-2.3 don't work with built-in Kotlin.
  - Re-drop the kotlin-android plugin from both build.gradle.kts files;
    AGP 9 enables built-in Kotlin by default and KSP 2.3 cooperates.
  - Remove android.builtInKotlin=false from gradle.properties.

compose-compiler plugin tracks the Kotlin version via version.ref, so
no separate bump there.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/build.gradle.kts

@@ -1,9 +1,9 @@
 plugins {
     alias(libs.plugins.android.application) apply false
-    // kotlin-android needed despite AGP 9's built-in Kotlin: KSP isn't
-    // compatible with the builtInKotlin path yet, so we opt out via
-    // android.builtInKotlin=false in gradle.properties.
-    alias(libs.plugins.kotlin.android) apply false
+    // kotlin-android NOT registered: AGP 9 built-in Kotlin + KSP 2.3.x
+    // covers the Kotlin compilation path. Language-level Kotlin compiler
+    // plugins (serialization, compose-compiler) are still applied
+    // explicitly per module.
     alias(libs.plugins.kotlin.serialization) apply false
     alias(libs.plugins.ksp) apply false
     alias(libs.plugins.hilt) apply false