chore(android): upgrade toolchain to AGP 8.13.1 / Kotlin 2.3.0 (#23)

Bump Android Gradle Plugin from 8.11.1 to 8.13.1 and Kotlin from
2.2.20 to 2.3.0 to align with newer plugin version requirements.

Kotlin 2.3.0 removed the kotlinOptions DSL free-string assignment.
Migrate to the compilerOptions DSL for JVM target configuration.

Gradle wrapper remains at 8.14 (compatible with AGP 8.13.1).

apps/chanora_flutter/android/app/build.gradle.kts

@@ -63,8 +63,10 @@ android {
         targetCompatibility = JavaVersion.VERSION_17
     }
 
-    kotlinOptions {
-        jvmTarget = JavaVersion.VERSION_17.toString()
+    kotlin {
+        compilerOptions {
+            jvmTarget.set(org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17)
+        }
     }
 
     defaultConfig {