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/gradle.properties

@@ -4,10 +4,4 @@ org.gradle.caching=true
 org.gradle.configuration-cache=true
 android.useAndroidX=true
 android.nonTransitiveRClass=true
-
-# KSP isn't compatible with AGP 9's built-in Kotlin support (the
-# `android.builtInKotlin=true` default). Opt out so we can apply the
-# explicit kotlin-android plugin and KSP together. Revisit when KSP
-# adds built-in-Kotlin support.
-android.builtInKotlin=false
 kotlin.code.style=official